Section 3 - Area Defense - InfantryDrills.com

5-105. An area defense is a type of defensive operation that concentrates on denying enemy forces access to designated terrain for a specific time rather than destroying the enemy outright (ADP 3-90). The focus is on retaining terrain where the bulk of the defending force positions (main battle area) itself in mutually supporting prepared positions. Defenders maintain their positions and control the terrain between these positions and focus their fires into engagement area (see paragraph 5-204), which reserve units can supplement. The leader uses the reserve force to reinforce fires, add depth, block penetrations, restore positions, counterattack to destroy enemy forces, or seize the initiative.

Organization Of Forces

5-106. The Infantry rifle platoon leader organizes an area defense around the static framework of the defensive positions seeking to destroy enemy forces by interlocking fire or local counterattacks. The platoon leader (in coordination with the company commander) has the option of selecting from the two methods of positioning units, defending in-depth or defending forward (see paragraphs 5-114 to 5-118). The platoon’s ability to defend in depth depends on the threat, task organization of the company, and nature of the terrain. When the leader defends forward within an AO, the force is organized so that most of the available combat power is committed early in the defensive effort. In an area defense, the leader organizes the defending force to accomplish information collection (especially reconnaissance and surveillance), security, main battle area, reserve, and sustainment missions.

5-107. The platoon normally conducts defensive operations within the Infantry rifle company as part of an Infantry battalion’s area defense to disrupt or destroy an enemy force, control terrain, or protect a friendly force. The orientation of the platoon defense is determined from the mission analysis. The platoon’s purpose in the mission statement clearly focuses the platoon on the enemy, the terrain, or a friendly force. The platoon may take part in many differing roles as part of a larger force: as part of the security force (in the security area, see paragraph 5-16) and main body (in the main battle area, see paragraph 5-17), conducting information collection (reconnaissance and surveillance, and security operations), occupying a battle position(s) in the main battle area, or as a reserve or part of a reserve. The platoon defense normally focuses on reconnaissance, surveillance, security, defensive operations, and actions by the reserve.

5-108. The platoon uses numerous security techniques in concert with deliberate well-planned defensive positions, incorporating such things as ambushes, reconnaissance, and security patrolling (movement to contact). The platoon leader analyzes how the terrain effects where the enemy can go and where to emplace forces from the platoon (within the security area and main battle area) that deny the enemy the ability to compromise the company and battalion defenses.

5-109. Platoon missions within the security area consist of security, reconnaissance, surveillance, and counter reconnaissance missions. Depending on the specific missions assigned, the platoon may simply observe and report, engage with indirect fires, and/or engage with direct fire (the least preferred) weapons. Security operations are crucial throughout the security area initially, to support the preparation of the defense; early in the fight, to disrupt the enemy attack and/or to identify the main effort; and in the main battle area, to support the higher commander’s decision-making process. The platoon leader considers activities that must be conducted in the security area that inhibit the enemy compromising the defense. Combat patrols, ambushes, security patrols, and raids should be considered in concert with the preparation of defensive positions. The combat power applied to these combat patrols are affected by METT-TC (I) and the higher commander’s intent.

5-110. Platoon missions conducted in the main battle area are oriented on enemy destruction, terrain retention, and force protection. Normally, the decisive fight occurs in the main battle area; therefore, the main effort is located there. Units tasked with security missions, supporting efforts, or reserve missions must support the main effort in the commander’s concept for conducting the defense.

Note. The reserve is used to seize and maintain the initiative. Although the reserve does not have an assigned mission that directly supports the main effort, the reserve is positioned to be employed at the decisive time and place to ensure the success of the defense.

5-111. The platoon uses numerous local security techniques in concert with deliberate well planned defensive positions, incorporating such things as ambushes, reconnaissance, and security patrolling (movement to contact). The platoon leader analyzes how the terrain effects where the enemy can go and where to emplace forces from the platoon (within the security area and main battle area) that deny—a task to hinder or prevent the enemy from using terrain, space, personnel, supplies, or facilities (ATP 3-21.20)—the enemy the ability to compromise the company and battalion defenses.

5-112. Sustainment missions in an area defense requires a balance between establishing forward supply stocks of ammunition, barrier material, and other supplies in sufficient amounts, and having the ability to move the supplies in conjunction with enemy advances. Maintenance and medical support, with their associated repair parts and medical supplies, must also be balanced between being employed forward and having the ability to move. Echeloned company trains (echeloned trains include battalion combat and field trains) provide the company commander with the flexibility to execute platoon sustainment missions in support of the area defense. Company trains(see chapter 8) are located between 500 and 1,000 meters away from forward platoon combat missions. By placing at least one terrain feature between it and the enemy, the company trains will be out of the enemy’s direct fire weapons. In some circumstances, it may be necessary to emplace cache’s forward of the company trains, and to the rear of the platoon battle positions or company command post in a defilade position, with cover when possible, for easier access throughout the duration of the defense.

5-113. A platoon may perform all these operations at the same time in its own AO; or it may be tasked to do one or more of them for a larger unit. (See ATP 3-21.10 and ATP 3-21.20 for additional information.)

Position Selection

5-114. The two methods for the positioning of units within an area defense are defense in-depth and forward defense. The Infantry platoon is expected to be able to do both. While the Infantry company commander usually selects the type of area defense to use, the higher commander often defines the general defensive scheme for the Infantry rifle company and platoon. The specific mission may impose constraints such as time, security, and retention of certain areas that are significant factors in determining how the rifle company and platoon will defend. Regardless of the method of positioning, platoon will typically defend from battle positions.

Defense In-Depth

5-115. Defense in-depth reduces the risk of the attacking enemy quickly penetrating the defense. The enemy is unable to exploit a penetration because of additional defensive positions employed in-depth. (See figure 5-4 on page 5-34.) The in-depth defense provides more space and time to defeat the enemy attack. Platoons can achieve depth in positions through planning supplementary or subsequent battle positions (see paragraphs 5-22 and 5-23, respectively). When platoons are tasked to defend, the expectation is that they plan for depth by planning for multiple battle positions in depth within their assigned AO.

5-116. The Infantry platoon uses a defense in-depth when—

The mission allows the platoon to fight throughout the depth of the AO.
The terrain does not favor a defense well forward, and better defensible terrain is available deeper in the AO.
Sufficient depth is available in the AO.
Cover and concealment forward in the AO are limited.
Weapons of mass destruction may be used.

Figure 5-4. Platoon defense in-depth

Forward Defense

5-117. The intent of a forward defense is to prevent enemy penetration of the defense. (See figure 5-5.) Due to lack of depth, a forward defense is least preferred. The platoon deploys most of its combat power into forward defensive positions near the forward edge of the battle area. While the company defense may lack depth, the platoon and squads must build depth into the defense at their levels. Achieving depth usually comes with a cost in terms of reduced frontage but is essential to an effective defense. The leader fights to retain the forward position and may conduct counterattacks against enemy penetrations or destroy enemy forces in a forward engagement area. Often, counterattacks are planned forward of the forward edge of the battle area to defeat the enemy.

5-118. The platoon uses a forward defense when—

Terrain forward in the AO favors the defense.
Strong existing natural or man-made obstacles, such as river or a rail line, are located forward in AO.
The assigned AO lacks depth due to location of the area or facility to be protected.
Cover and concealment in rear portions of the AO are limited.
Directed by higher headquarters to retain or initially control forward terrain.

Figure 5-5. Platoon forward defense

Variations Of The Area Defense

5-119. The Infantry rifle platoon usually defends using one of three variations of the area defense: defense of a linear obstacle, perimeter defense, or reverse slope. The platoon also can defend using a combination of these variations. (See ATP 3-21.10 for additional information.)

Defense Of A Linear Obstacle

5-120. A platoon may conduct a defense of a linear obstacle along or behind a linear obstacle within an area defense. Linear obstacles such as mountain ranges or river lines generally favor a forward defense. It is extremely difficult to deploy in strength along the entire length of a linear obstacle. The defending leader must conduct economy of force measures in some areas.

5-121. Within an area defense, the leader’s use of a defense in-depth accepts the possibility the enemy may force a crossing at a given point. The depth of the defense should prevent the enemy from rapidly exploiting its success. It also diffuses the enemy’s combat power by forcing the enemy to contain bypassed friendly defensive positions in addition to continuing to attack positions in greater depth.

5-122. This variation of an area defense may be used when defensible terrain is available in the forward portion of the platoon’s AO, or to take advantage of a major linear natural obstacle. This technique allows interlocking and overlapping observation and fields of fire across the platoon’s front. (See figure 5-6.) The bulk of the platoon’s combat power is well forward. Sufficient resources must be available to provide adequate combat power to detect and stop an attack. The platoon relies on fighting from well-prepared mutually supporting positions. It uses a high volume of direct and indirect fires to stop the attacks. The main concern when fighting this variation of an area defense is the lack of flexibility and the difficulty of both seizing the initiative, seeking out enemy weaknesses, and the difficulty of developing depth within the defense. Obstacles, indirect fires, and contingency plans are vital to this maneuver. The platoon depends upon surprise, well-prepared positions, and deadly accurate fires to defeat the enemy. The platoon will normally not have the resources to establish a separate reserve, the company’s reserve in this case is generally small, and perhaps a squad from one of the three platoons.

Figure 5-6. Defense of a linear obstacle (mutual support between squads)

5-123. Minefields and other obstacles are positioned and covered by fire to slow the enemy and inflict casualties. Engaging the enemy at long-range by supporting fires disrupts the momentum of the enemy’s attack. Use fires from mortars, machine guns, and small arms as the enemy comes into range. If the defense is penetrated, block the advance with the reserve and shift fire from the forward squads onto the enemy flanks. Then, counterattack with the reserve or a less committed squad with intense fires. The purpose is to destroy isolated or weakened enemy forces and regain key terrain.

5-124. The counterreconnaissance effort is critical when fighting to deny the enemy the locations of the platoon’s forward positions. If the enemy locates the forward positions, the enemy will concentrate combat power where desired while fixing the rest of the platoon to prevent their maneuver to disrupt the enemy attack. This effort might be enhanced by initially occupying and fighting from alternate positions (see paragraph 5-22) forward of the primary positions. This tactic enhances the security mission and deceives the enemy reconnaissance that may get through the security force.

Perimeter Defense

5-125. A perimeter defense is a defense oriented in all directions. (See figure 5-7 on page 5-38.) The platoon uses it for self-security, and to protect other units located within the perimeter. The platoon can employ a perimeter defense in urban or woodland terrain. The platoon might be called upon to execute the perimeter defense under a variety of conditions including:

When it must conserve or build combat power to execute offensive operations or patrolling missions.
When it must hold critical terrain in an AO.
When its defense is not tied in with adjacent units.
When it has been bypassed and isolated by the enemy and must defend in place.
When it is used in a reserve position, in a TAA or patrol base, on a follow-on decentralized platoon operation during resupply or when the platoon is otherwise isolated.
When it is directed to concentrate fires into two or more adjacent avenues of approach.

Figure 5-7. Perimeter defense (squads positioned in-depth)

Note. The actions addressed in paragraph 5-126 will generally apply to the defense of a linear obstacle and reverse-slope defense.

5-126. Preparing a perimeter defense is like preparing any other position defense, but the platoon must disperse in a circular configuration for all-round security. The actual shape depends on the terrain. The following actions constitute setting up a perimeter defense:

The platoon leader assigns squads to cover the most likely approach and prepares alternate and supplementary positions within the perimeter.
Javelins (or M3 Multi-role, Antiarmor, Antipersonnel Weapon System [known as MAAWS]) cover likely armor approaches.
The platoon may use hide positions and move forward to fire as the enemy appears. The platoon leader assigns several firing positions. If there are few positions for them, they are assigned a primary position and are dug in.
Snipers (when attached) or squad designated marksman should cover likely or suspected enemy positions or observation posts.
Snipers (when attached) and squad designated marksmen also should be used to observe or overwatch areas where civilians congregate.
Keep attached mortars near the center of the perimeter so their minimum range does not restrict their ability to fire in any direction.
Mortars should dig in and have covered ammunition storage bunkers.
If possible, hold one or more rifle teams in reserve.
The platoon leader assigns a primary position to the rear of the platoon, covering the most dangerous avenues of approach, and may assign the rifle squad supplementary positions since the platoon is prepared to fight in all directions.
Prepare obstacles in-depth around the perimeter.
Plan direct and indirect fire as for any type of defense.
Plan and use direct and indirect fire support from outside the perimeter when available.
Counter enemy probing attacks by area fire weapons (artillery, mortars, claymores, and grenade launchers) to avoid revealing the locations of fighting positions (rules of engagement [ROE]-dependent).
If the enemy penetrates the perimeter, the reserve destroys, and then blocks the penetration:
- It also covers friendly Soldiers during movement to alternate, supplementary, or subsequent positions.
- Even though the platoon’s counterattack ability is limited, it must strive to restore its perimeter.
Sustainment elements may support from within the perimeter or from another position.
Supply and evacuation might be by air. Consider the availability of landing zones and drop zones (protected from enemy observation and fire) when selecting and preparing the position.
Position machine gun teams with principal direction of fires (known as PDFs) or FPLs. (Build the defense by first positioning key weapons, and then squads/teams.)
Position riflemen where they can protect machine guns.
Identify dead space and position grenadiers.
Position riflemen with depth, overlap/interlock fires, and stand off to grenade range.
Ensure gaps/seams are closed between elements through adjacent unit coordination (left-to-right, front-to-rear).

5-127. The Y-shaped perimeter defense is a variation of the perimeter defense that uses the terrain effectively. This defense is used when the terrain, cover and concealment, or fields of fire do not support the physical positioning of the squads in a circular manner. The Y-shaped perimeter defense is so named because the squad’s battle positions are positioned on three different axes radiating from one central point. (See figure 5-8 on page 5-40.)

Figure 5-8. Y-shaped perimeter defense (primary and alternate engagements)

5-128. The Y-shaped variation is still a perimeter defense because it is effective against an attack from any direction. The Y-shaped defense provides all-round perimeter fires without having to position Soldiers on the perimeter. It is likely to be most effective in mountainous terrain, but it also may be used in a dense jungle environment due to limited fields of fire. All the fundamentals of a perimeter defense previously discussed apply, with the following adjustments and special considerations:

Although each squad battle position has a primary orientation for its fires, each squad must be prepared to reorient to mass fires into the engagement areas to its rear.
When there is no identified most likely enemy approach, or in limited visibility, each squad may have half its Soldiers oriented into the engagement areas to the front and half into the engagement areas to the rear. Ideally, supplementary individual fighting positions are prepared, allowing Soldiers to reposition when required to mass fires into one engagement area.
When a most likely enemy avenue of approach is identified, the platoon leader may adjust the normal platoon orientations to concentrate fires (see figure 5-9) for the following reasons:
- This entails accepting risk in another area of the perimeter.
- The platoon security plan should compensate for this with additional observation posts, patrols, or other measures.
The positioning of the platoon command post, reserve, or any sustainment assets is much more difficult due to a lack of depth within the perimeter.

Figure 5-9. Modified Y-shaped perimeter defense

5-129. The most difficult aspect of the Y-shape perimeter defense is the fire control measures required. To fight this defense without casualties from fratricide, the leaders must ensure the limits of fire for each weapon do not allow fires into the adjacent squad’s positions. In a mountainous environment, firing downward into the engagement area may make this simpler. Some measures to consider include:

Position medium machine guns near the apex of the “Y” to allow an FPL that covers the platoon front while firing away from the adjacent units.
Cover the areas of the engagement areas closest to the apex with claymores, non-persistent mines, or obstacles to reduce the need for direct fires in these areas.
Identify those positions at most risk to fratricide and prepare the fighting position to protect the Soldier from fires in this direction.
The loss of one squad position may threaten the loss of the entire platoon. To prevent this, plan and rehearse immediate counterattacks with a reserve or the least committed squad.
Consider allowing the enemy to penetrate well into the engagement areas and destroy the enemy as in an ambush.
Be aware that if a Y-shape defense is established on the prominent terrain feature and the enemy has the ability to mass fires, the enemy may fix the platoon or squad with direct fires and destroy it with massed indirect fires.

Reverse-Slope Defense

5-130. An alternative to defending on the forward slope of a hill or a ridge is to defend on a reverse slope. (See figure 5-10.) In such a defense, the platoon is deployed on terrain that is masked from enemy direct fire and ground observation by the crest of a hill. Although some units and weapons might be positioned on the forward slope, the crest, or the counterslope (a forward slope of a hill to the rear of a reverse slope), most forces are on the reverse slope. The key to this defense is control the crest by direct fire.

Figure 5-10. Reverse-slope defense (slope terminology)

5-131. Another reverse-slope variation available when line of sight restrictions exist to a unit’s direct front is to organize a system of reverse slope defenses firing to the oblique defilade, each covering the other. (See figure 5-11.) In this example, platoon main battle area positions were unable to engage targets directly to their front but could cover each other using oblique defilade. Line of sight restrictions can be obstacles, terrain, and vegetation driven. This system of reverse slope defenses protects defenders from enemy frontal and flanking fires and from fires coming from above the main defensive area.

Figure 5-11. Reverse-slope defense (oblique defilade)

5-132. To succeed, the platoon leader prevents the attacker from conducting a detailed observation of the reverse-slope defense through the deployment of aggressive patrols (combat and reconnaissance [see chapter 7]) and security operations (see chapter 6 section VII) forward of the topographical crest. The platoon establishes observation posts and combat outposts on, or forward of the topographical crest. This allows long-range observation over the entire front, and indirect fire coverage of forward obstacles. Observation posts may vary in size and should include FOs. During limited visibility, the numbers may increase to improve security. Additional planning fundamentals to a defense on a reverse slope include:

Positioning forward squads so they block enemy approaches and exploit existing obstacles. Plans should—
- Permit surprise fire on the crest and on the approaches around the crest.
- Have rear and OHC to protect friendly Soldiers from fratricide while in forward fighting positions.
Positioning observation posts on the crest or the forward slope of the defended hill. Plans should—
- Increase observation posts and patrols to prevent infiltration at night.
- Consider attaching medium machine guns to observation posts.
Positioning the squad in-depth or reserve where it can provide the most flexibility, support the forward squads by fire, protect the flanks and the rear of the platoon, and counterattack, if necessary. It might be positioned on the counterslope to the rear of the forward squad if that position allows it to fire and hit the enemy when the enemy reaches the crest of the defended hill.
Positioning the platoon command post to the rear where it will not interfere with the reserve or supporting units. Plans should consider that—
- The platoon leader may have an observation post on the forward slope or crest and another on the reverse slope or counterslope.
- The observation post is used on the forward slope or crest before the battle starts when the platoon leader is determining the enemy’s intentions.
- During the fight, the platoon leader moves the observation post on the reverse slope or counterslope.
Planning indirect fire well forward of, on, and to the flanks of the forward slope, crest, reverse slope, and counterslope.
Planning FPFs on the crest of the hill to control the crest and stop assaults.
Reinforcing existing obstacles.
Planning protective obstacles on the reverse slope—just down from the crest where it can be covered by fire—can slow the enemy’s advance and hold the enemy under friendly fire.
Planning for counterattacks and plans to drive the enemy off the crest by fire, if possible.
Planning to drive the enemy off by fire and movement.

5-133. The platoon leader may position all or part of the defending platoon to employ a reverse-slope defense. This technique allows subordinate units to concentrate direct fires into a relatively small area while being protected from the enemy’s direct observation and supporting fires. The defender can destroy the enemy that is isolated forward through surprise and concentrated fires. Maintaining control of the forward slope is essential for success. Control of the forward slope is accomplished by using dominating terrain behind the defender or by using stay behind forces, such as an observation post, which can observe and call for fire on the attacker. The platoon leader may choose to employ a reverse slope defense when—

Enemy fire makes the forward slope untenable.
Lack of cover and concealment on the forward slope makes it untenable.
The forward slope has been lost or not yet been gained.
The forward slope is exposed to enemy direct fire weapons fired from beyond the effective range of the defender’s weapons. Moving to the reverse slope removes the attacker’s standoff advantage.
The terrain on the reverse slope provides better fields of fire than the forward slope.
Surprising and deceiving the enemy as to the true location of the platoon’s defensive positions is essential.
Enemy weapon systems have overmatch in range and lethality.

5-134. When executing a reverse slope defense, it is important for the platoon leader to consider emplacing overwatching elements forward of the topographic crest and on the flanks to protect the main defensive positions of the defending platoon. This is especially desirable when overwatching elements can observe and place fires on the crest and forward slope. Additional areas of special emphasis include:

A direct and indirect fire support plan to prevent the enemy’s occupation and using the crest of the hill.
The use of observation posts or reconnaissance elements on the forward slope to provide observation across the entire front and security to the main battle positions.
A counterattack plan specifying measures necessary to clear the crest or regain it from the enemy.
Fire support to destroy, disrupt, and attrit enemy forces on the forward slope.

5-135. The forward edge of positions should be at the maximum range allowable by the available fields of fire allowing the defender time to place well-aimed fire on the enemy before the enemy reaches friendly positions. The platoon establishes observation posts on or forward of the topographical crest. This allows long range observation over the entire front and indirect fire coverage of forward obstacles. Observation posts usually are provided by the unit owning the terrain being observed and may vary in size from a few Soldiers to a reinforced squad. They should include FOs. At night, their number should be increased to improve security. Additional considerations the platoon leader may apply when defending on a reverse slope include:

Observation of the enemy is more difficult.
Soldiers in this position see forward no farther than the crest. This makes it hard to determine exactly where the enemy is as they advance, especially when visibility is poor.
Observation posts must be placed forward of the topographic crest for early warning and long-range observation.
Withdraw from the position might be more difficult.
Fields of fire are usually short.
Obstacles on the forward slope can be covered only with indirect fire or by units on the flanks of the platoon unless some weapon systems are placed forward initially.
If the enemy gains the crest, they can assault downhill. This may give the enemy a psychological advantage.
If observation posts are insufficient or improperly placed, the defenders might have to fight an enemy who suddenly appears in strength at close range.
It is exceedingly difficult to break contact meaning that a reverse slope engagement is decisive resulting in one or both forces being severely attritted.
The attacking force retains the initiative as long as it stays behind the crest of the front slope.

Area Defense Sequence Of Events, Example

5-136. The Infantry rifle platoon may assume a defensive mission following an attack of its own or in anticipation of an enemy attack. Most defenses will follow a sequence of events similar to the example sequence addressed in paragraphs 5-137 to 5-173, as part of a larger defense. This sequence of events is used for discussion purposes and is not the only way to sequence an area defense. Some events may be performed simultaneously or in an order different from shown in the example sequence. For example, events (such as reconnaissance and surveillance, security, and occupation)may recur several times during the conduct of defense. With any defense, the platoon leader and platoon subordinate leaders understand events will vary depending on the mission variables of METT-TC (I) and events will overlap.

Leader’s Reconnaissance

5-137. Before occupying any position, to include those in the forward security area, leaders at all echelons conduct reconnaissance (building on the company commander’s reconnaissance plan) of their assigned position(s). The reconnaissance effort is as detailed as possible regarding the mission variables of METT-TC (I). Reconnaissance can consist of a simple map reconnaissance, or a more detailed leader’s reconnaissance and initial layout of the new position. When feasible, the platoon leader and subordinate leaders conduct a leader’s reconnaissance of the complete AO to develop plans based on their view of the actual terrain. When available and dependent upon the situation, aviation assets may be used to conduct the leader’s reconnaissance.

5-138. An effective leader’s reconnaissance allows the platoon to rapidly occupy defensive positions and immediately start priorities of work. Participants in the platoon’s reconnaissance include (the company commander when possible) the platoon leader, subordinate squad leaders, and security personnel. The goals are, but not limited to, identification of enemy avenues of approach, engagement areas, sectors of fire, the tentative obstacle plan, indirect fire plan, observation post locations, command post locations, mortar firing point, and logistics element location. The platoon leader develops a plan for the leader’s reconnaissance that includes the following:

Provisions for security.
Areas to reconnoiter.
Priorities and time allocated for the reconnaissance.
Considerations for fire support, communications, and CASEVAC and MEDEVAC plans.
Contingency plan if the reconnaissance is compromised.

Establish Security

5-139. When the Infantry rifle platoon is part of a larger unit’s area defense, the higher-level commander (for example, the company or battalion) establishes a forward security area before the unit moves to defend. Even with this forward security area established, subordinate platoons of the company must still provide for their own local security, especially over large geographical areas or in complex terrain and noncontiguous assigned area.

Security Operations Within the Security Area

5-140. In order to prevent the enemy from observing and interrupting defensive preparations and identifying unit positions, the higher-level commander (normally the battalion commander) establishes the security area well forward of the planned main battle area for the battalion, but within indirect fire and communications range. When the battalion commander is unable to push the security area forward to achieve this objective, the battalion may have to hold its positions initially, as it transitions and then withdraws units to the defensive main battle area, establishing a forward security force in the process. The rifle platoon may be called upon to act as anyone of these forces in the conduct of the defense.

5-141. In a contiguous assigned area, the battalion commander normally organizes and defines the security area forward of the forward edge of the battle area and assigns company assigned area to prevent gaps in the battalion area defense. In a noncontiguous assigned area, the battalion commander normally organizes and defines the security area forward of the main battle area, or along likely avenues of approach. In a noncontiguous assigned area, individual companies will have more responsibility for independent security area actions. The platoon leader ensures the independent security area actions at all levels within the platoon align with the security area plan for the company.

Security Operations Within the Main Battle Area

5-142. As the security forces forward of the battalion’s main battle area are arrayed, the company commander and platoon leaders plan security operations within the main battle area, to prevent enemy reconnaissance, reduction of obstacles, targeting of friendly positions, and other disruptive actions. Subordinate platoons and squads secure obstacles, battle positions, command posts, and sustainment sites (company trains) throughout the AO.

Active and Passive Measures to Avoid Detection

5-143. Security in the defense includes all active and passive measures taken to avoid detection by the enemy, deceive the enemy, and deny enemy reconnaissance elements accurate information on friendly positions. The two primary tools available to the platoon leader are observation posts and patrols. In planning for security in the defense, the platoon leader considers the military aspects of terrain observation and fields of fire, avenues of approach, key terrain, obstacles and cover, and concealment. The leader uses the map to identify terrain that will protect the platoon from enemy observation and fires, while providing observation and fires into the engagement area. The leader uses intelligence and combat information updates to increase situational understanding, reducing the possibility of the enemy striking at a time or in a place for which the platoon is unprepared.

5-144. An observation post provides the primary security in the defense. Observation posts provide early warning of impending enemy contact by reporting direction, distance, and size. It detects the enemy early and sends accurate reports to the platoon. The platoon leader establishes observation posts along the most likely enemy avenues of approach into the position or into the AO. Leaders ensure that observation posts (normally dismounted) have communication with the platoon.

5-145. Early detection reduces the risk of the enemy overrunning the observation post. Observation post may be equipped with a Javelin command launch unit (known as CLU); class 1 unmanned aircraft system; seismic, acoustic, or frequency detecting sensors to increase its ability to detect the enemy. They may receive infrared trip flares, infrared parachute flares, infrared M320 rounds, and even infrared mortar round support to illuminate the enemy. The platoon leader weighs the advantages and disadvantages of using infrared illumination when the enemy is known to have night vision devices that detect infrared light. Although infrared and thermal equipment within the platoon enables the platoon to see the observation post at a greater distance, the platoon observation post should not be positioned beyond the supporting range of the platoon’s small-arms weapons.

5-146. To reduce the risk of fratricide further, observation posts use Global Positioning Systems (GPSs) to navigate to the exit and entry point in the platoon’s position. The platoon leader submits an observation post location to the company commander to ensure a no fire area is established around each observation post position.

5-147. Platoons actively patrol in the defense. Patrols enhance the platoon’s ability to fill gaps in security between observation posts. The platoon leader forwards the tentative patrol route to the commander to ensure it does not conflict with other elements within the company. The commander forwards the entire company’s patrol routes to the battalion. This allows the operations and intelligence staff officers to ensure all routes are coordinated for fratricide prevention and no gaps are present. The patrol leader may use a GPS to enhance basic land navigational skills as the leader tracks the patrol’s location on a map.

5-148. During the defense, the platoon leader will either set a security level or comply with the security posture established by the company commander, this increases or decreases depending on enemy situation. For example, 25 percent security, 50 percent security, or up to 100 percent security at stand-to (30 minutes before and after begin morning nautical twilight or end evening nautical twilight) or when the situation dictates. Local security patrols continually inspect the integrity of emplaced obstacles and clear dead space and identify signs of enemy reconnaissance activities within the platoon’s AO. Unit alert plans ensure all positions are check periodically, observation posts are relieved periodically, and at least one leader (more depending on the situation) is always alert.

Occupation And Priorities Of Work

5-149. When the battalion or other higher echelon establishes a security area, a subordinate company or platoon within the battalion may be identified as a security force to deploy forward as remaining forces occupy and prepare positions in the main battle area. Security of the main battle area is critical during occupation to ensure units within the main battle area avoid detection and maintain combat power for the actual engagements with the defense.

Plan of Occupation

5-150. The plan of occupation for the platoon must be thoroughly understood to maximize the time available for occupation and preparation of the defense. When occupying/establishing the security area, the battalion commander may lead with the scout platoon to conduct reconnaissance and establish observation post along the forward edge of the security area. Company commanders must ensure that subordinate forces understand security force locations forward of the main defenses.

5-151. Within the security area, platoons within the company may be called to assist or augment forward security forces from the scout platoon or conduct security force operations in support of the battalion. During occupation of the security area, security forces establish observation posts forward, and from these observation posts, the security forces use long-range fires to hinder the enemy’s preparations, to reduce the force of the enemy’s initial blows, and to start the process of wresting the initiative from the enemy. Throughout the security area, security forces position and reposition (though most movement should be limited to avoid detection) to—

Prevent enemy observation of defensive preparations.
Defeat infiltrating enemy reconnaissance forces.
Prevent the enemy from delivering direct fires or observed indirect fires into the battalion area defense.
Provide early warning of the enemy’s approach.

5-152. Within the main battle area, and depending on the situation, commanders at all levels may send a subordinate force to initially secure positions prior to the main body’s arrival. The mission of this force is to continue to conduct reconnaissance and surveillance of key terrain and obstacles, guide and provide local security as the defenses main body occupies the defense and initiates priorities of work. As all elements within the main battle area establish local security, priorities of work continue and includes but is not limited to refining the plan, positioning of forces, preparing positions, constructing obstacles, planning and synchronizing fires, positioning logistics, and conducting inspections and rehearsals. To aid in operations security and to reduce vulnerability, the platoon leader analyzes the benefits of dispersion against the requirements and resources for the security area. Usually, the greater the dispersion between platoons (or companies within the battalion defense) the larger the security area.

Preparation and Conduct of a Defense

5-153. Priority of work is a set method of controlling the preparation and conduct of a defense. Tactical SOPs should describe priority of work including individual duties. The platoon leader changes priorities based on the situation. All leaders in the platoon should have a specific priority of work for their duty position. Although listed in sequence, several tasks are performed at the same time. An example leader and Soldier priority of work sequence is as follows:

Post local security.
Set and then maintain a designated level of security.
Establish the platoon’s reconnaissance and surveillance.
Position Javelins (or M3 MAAWS) and machine guns (designate FPLs and FPFs); assign sectors of fire.
Position subordinate units and other assets.
Prepare hasty fighting positions.
Establish platoon command post.
Clear fields of fire and prepare standard range cards and sector sketches.
Adjust indirect fire FPFs. The firing unit fire direction center should provide a safety box clearing of all friendly units before firing adjusting rounds.
Emplace early warning devices.
Modify hasty fighting positions to deliberate fighting positions as the enemy situation allows.
Install wire communications, if applicable.
Emplace obstacles and mines.
Mark (or improve marking for) target reference points (TRPs) and direct fire-control measures.
Improve primary fighting positions such as OHC.
Prepare alternate and supplementary positions.
Conduct maintenance (specifically, weapons and communications equipment).
Establish sleep and rest plan.
Reconnoiter movements and routes (for example, trafficability and timing).
Rehearse engagements and disengagements or displacements.
Adjust positions and control measures as required.
Stockpile ammunition, food, and water.
Dig trenches between positions.
Continue to improve positions (for example, replaced camouflage concealment if it changes color or dries out).

Note. Leaders carefully manage security levels to achieve priorities of work.

Duties and Areas of Responsibilities During Defensive Preparation

5-154. Many platoon leader duties and responsibilities during the defense can be delegated to subordinates, but the platoon leader ensures they are done. This includes:

Ensuring local security and assigning observation post responsibility.
Conducting a leader’s reconnaissance with the platoon sergeant and selected leaders.
Confirming or denying significant deductions or assumptions from the mission analysis.
Understanding/confirming the initial terrain analysis (for example, avenues of approach, potential gaps and seams, and dead space) made prior to arrival.
Confirming/revising the direct fire plan, to include engagement area, sectors of fire, position key weapons, and fire control measures based on the reality of the terrain.
Designating primary, alternate, supplementary, and subsequent positions supporting the direct fire plan, for platoons, sections, and supporting elements.
Requiring squads to conduct coordination.
Integrating indirect fire plan and obstacles to support the direct fire plan.
Designating the platoon command post location (for example, collocating with a squad) and positioning key weapons.
Checking the platoon command post and briefing the platoon sergeant on the situation and logistics requirements.
Upon receipt of the squads’ sector sketches, making two copies of the platoon defensive sector sketch and fire plan, retaining one copy, and forwarding the other copy to the company. (See figure B-7 on page B-13 for an example of a platoon defense sector sketch.)
Confirming the direct fire plan and squad positions before digging starts.
Coordinating with the left and right units.
Checking with the company commander for all changes or updates in the orders.
Finishing the security, deception, counterattack, and obstacle plans.
Walking the platoon positions after they are dug.
Confirming clear fields of fire and complete coverage of the platoon’s entire AO by all key weapons.
Looking at the defensive plan from an enemy point of view, conceptually and physically.
Checking dissemination of information, interlocking fires, and dead space.
Ensuring immediate correction of deficiencies.
Ensuring rehearsals are conducted and obstacle locations reported.
Ensuring routine report status of and completion of defensive preparations to the company.

5-155. Platoon sergeant’s duties and responsibilities include:

Establishing the platoon command post and ensures wire communications link the platoon, squads, and attached elements, if applicable.
Establishing CCPs, platoon logistics release points (known as LRPs), and detainee collection points, and locating company-level points.
Briefing squad leaders on the platoon command post location, logistics plan, and routes between positions.
Assisting the platoon leader with the sector of fire and sector sketch.
Requesting and allocating pioneer tools, barrier materiel, rations, water, and ammunition.
Walking the positions with the platoon leader. Supervising emplacement of squads, key weapons, check standard range cards, and sector sketches.
Establishing routine security or alert plans, radio watch, rest plans, and briefing the platoon leader.
Supervising continuously and assisting the platoon leader with other duties as assigned.
Selecting a straddle trench location and ensuring it is properly marked (see ATP 4-25.12).

5-156. The squad leader—

Emplaces local security.
Confirms positioning and assigned sectors of fire within the squad.
Confirms positioning and assigned sectors of fire for the CCMS and medium machine gun teams.
Positions and assigns sectors of fire for automatic rifleman, grenadiers, and riflemen.
Establishes command post and wire communications.
Confirms designated FPL and FPFs (for example, changes to locations, directions, or systems).
Clears fields of fire and prepares standard range cards.
Prepares squad standard range card and sector sketch.
Digs fighting positions.
Establishes communication and coordination within the platoon and adjacent units.
Coordinates with adjacent units. Reviews sector of fire and sector sketch.
Emplaces AT and claymores, then wire and other obstacles.
Marks or improves marking for TRPs and other fire control measures.
Improves primary fighting positions and adds OHC (stage 2, see paragraph 5-237).
Prepares supplementary and alternate positions (same procedure as the primary position).
Establishes sleep and rest plans.
Distributes and stockpiles ammunition, food, and water.
Digs trenches to connect positions.
Continues to improve positions, construct revetments, replace camouflage, and add to OHC.

5-157. The FO—

Assists the platoon leader in planning the indirect fires to support defensive missions.
Advises the platoon leader on the status of all firing units, and on the use of obscurants or illumination.
Coordinates with the Infantry company fire support officer, firing units, and squad leaders to ensure the fire plan is synchronized and fully understood.
Ensures the indirect fire plan is rehearsed and understood by all.
Ensures all FPFs are adjusted as soon as possible.
Develops an observation plan.
Coordinates and rehearses all repositioning of observers within the platoon AO to ensure they can observe targets or areas of responsibility.
Develops triggers.
Reports information collection activities.
Ensures redundancy in communications.

Adjacent Unit Coordination

5-158. The goal of adjacent unit coordination is to ensure unity of effort in accomplishment of the platoon mission. Items adjacent units coordinate include—

Unit positions, including locations of vital leaders’ call signs and frequencies.
Locations of observation posts and patrols.
Overlapping fires (to ensure direct fire responsibility is clearly defined).
Coverage of exploitable gaps or seams between units.
TRPs (to ensure identification/coordinated method of target handoff).
Coverage of terrain features (roads, trails, and dead space) that lay along boundaries.
Alternate, supplementary, and subsequent battle positions.
Indirect fire information.
Obstacles (location and type).
Air defense considerations, if applicable.
Routes to be used during occupation and repositioning.
Sustainment considerations.

Coordination

5-159. In the defense, coordination ensures that units provide mutual support and interlocking fires. In most circumstances, the platoon leader conducts face-to-face coordination (the preferred method due to reduced signal emissions) to facilitate understanding and to resolve issues effectively. However, when time is extremely limited, digital coordination may be the only means of sending and receiving this information. The platoon leader should send and receive the following information using the radio or command and control system before conducting face-to-face coordination:

Location of key leaders.
Location of fighting positions.
Location of observations posts and withdrawal routes.
Location and types of obstacles.
Location of contact points—in land warfare, points on the terrain, easily identifiable, where two or more units are required to make contact (JP 3-50). Point control measures—designated military symbols are generally numbered from left to right.
Location of coordination points—are points that indicate a specific location for the coordination of tactical actions between adjacent units (FM 3-90).
Location, activities, and passage plan for scouts and other units forward of the platoon’s position.
Platoon’s digital sector sketch.
Location of all Soldiers and units operating in and around the platoon’s AO.

5-160. Current techniques for coordination hold true for units that are digitally equipped. If a digitized and a nondigitized unit are conducting adjacent unit coordination, face-to-face is the preferred method. The leader of the digitized unit has the option to enter pertinent information about the nondigitized unit into command and control systems for later reference. The digitally equipped platoon leader should show the adjacent unit leader the digital sector sketch. If face-to-face coordination is not possible, leaders share pertinent information by radio.

Note. The fictional scenario, used as discussion vehicles in figures 5-12 and 5-13 on pages 5-55 and 5-57, illustrate different ways an Infantry rifle platoon can accomplish its mission within the conduct of a higher echelon’s area defense. These two examples focus on potential challenges confronting the platoon leader in establishing a battle position within its company’s area defense but are not intended to be prescriptive of how the Infantry rifle platoon performs any particular operation. This same scenario drives the area defense techniques used in ATP 3-21.10 and ATP 3-21.20. ATP 3-21.8 focuses on the techniques used to perform missions, functions, or tasks in support of the Infantry rifle Company B along the enemy avenue of approach in COA 1 in both figures.

Security Area Engagement

5-161. When the platoon is part of the higher echelon’s security area defense, the platoon leader integrates engagements within the higher echelon’s defense. Within the security area defense, planned fires usually include FOs and fire support teams (FISTs)executing indirect fire targets on a primary enemy avenue of approach. Security forces within the defense can come in the form of the scout platoon, sniper squad, subordinate units within the Infantry rifle company, or a combination of all. (See figure 5-12.) This can be in support of the higher headquarters’ scheme of fires using IBCT or higher echelon artillery, or in support of the battalion, and company scheme with the use of organic mortars and allocated artillery fires.

Figure 5-12. Area defense (security area engagements), example

5-162. The scheme of fires within the security area combined with the use of tactical obstacles serve to disrupt the enemy and canalize the enemy in the engagement areas, and to force the enemy to commit enemy engineer assets prior to the main battle area engagement. Tactical obstacles (for example, situational obstacles) and fire support tasks are planned and triggered relative to specific enemy COAs. They are essential, allowing for more effective engagements within the security area. Forward security forces employed forward may cover these situational obstacles with direct fires or indirect fires prior to their withdrawal to positions within the main battle area.

5-163. A platoon may support a higher echelon or its own scheme of maneuver by fighting a delay through the depth of the security area and into the main battle area. The purpose may be to take advantage of restrictive avenues of approach, to set the conditions for a counterattack, or to avoid a decisive engagement until favorable conditions are set. As security forces complete the rearward passage of lines, main battle area forces assume control of the battle at the BHL. Battle handover from forward security forces to forward main battle area forces requires firm, clear arrangements—

For assuming command of the action.
For coordinating direct and indirect fires.
For the security force’s rearward passage of lines.
For closing lanes in obstacles.
For movement of the security force with minimal interference to main battle area actions.

5-164. As security area engagements transition into the main battle area, security area forces withdraw to battle positions within the main battle area and counterattack or reserve positions. Security area forces may remain in place to continue to attrit the enemy as it continues on its avenue of approach or may move to a flank or to the rear of the main battle area to provide security. (See ATP 3-21.10 for additional information.)

Main Battle Area Engagement

5-165. The platoon leader seeks to defeat, disrupt, or neutralize the enemy’s attack forward of or within the main battle area. The platoon leader integrates direct and indirect fires with the obstacle plan, local counterattacks, and, when established, reserve forces to destroy the enemy in designated engagement areas or to force the enemy transition to a retrograde or hasty defense. The leader uses planned engagement areas and battle positions to attack the enemy throughout the depth of the defense. Direct fire control measures allow the platoon to focus, shift, or distribute their fires as necessary. However, fire support may be limited to critical points and times. Control measures allow the leader to rapidly concentrate the use of combat power at the decisive point(s), provide flexibility to respond to changes, and allocate responsibility of terrain and obstacles to synchronize the employment of combat power.

5-166. As attacking forces reach the forward edge of the battle area, the enemy will try to find weak points in the defense and attempt to force a passage, possibly by a series of probing attacks. Forward elements engage the enemy’s lead forces as the enemy attack develops along identified enemy avenues of approach. (See figure 5-13.) The platoon arrays forces and establishes engagement areas using obstacles and fires to canalize enemy forces. When shaping efforts allow for the canalization of the enemy, the enemy advance slows, and the increased density of forces present good targets for defensive fires within engagement areas. The maximum effects of these simultaneous and sequential fires, brought to bear at this stage of the battle, enable the destruction of the attacking enemy force. Additional considerations and key leader duties may include:

Focusing and shifting fires between direct fire control measures depending on enemy movement and controlling rates of fire.
Decisions on displacement to alternate, subsequent, and supplementary positions.
Reporting enemy situation and alerts to emerging threats, opportunities, or changing nature of the situation.
Reaction to enemy fires (for example, artillery and/or aviation) and CBRN attacks.
Reports to higher, monitoring stockage levels, and cross leveling or resupply.
CASEVAC and MEDEVAC procedures.
Criteria to commitment the reserve.

Figure 5-13. Main battle area (platoon engagements), example

Follow Through

5-167. During the planning for the defensive operation, the platoon leader must discern from the company OPORD what the potential follow-on missions are and begin to plan how to achieve them. During this planning, the leader determines the possible timeline and location for defeat in detail, consolidate, reorganize, and transition which best facilitates future operations and provides adequate protection.

5-168. The platoon leader and platoon subordinate leaders plan and prepare for consolidate during TLP. The following actions are usually a part of consolidate:

Eliminate remaining enemy resistance in battle positions, noncontiguous assigned areas, and the security area.
Reestablish, reoccupy fighting positions and adjust unit boundaries.
Reestablish local security including OPs and patrols to deny enemy direct or indirect fire on defensive positions.
Continue to improve security by conducting other necessary defensive actions. These defensive actions include engagement area development, direct fire planning, and battle position preparation.
Adjust FPFs and register targets along likely mounted and dismounted avenues of approach.
Close gaps or lanes in obstacles; repair or replace damaged or breached obstacles and replace expended mines.
Secure enemy detainees.
Prepare for enemy counterattack.

5-169. Reorganization usually is conducted concurrently with consolidate. It consists of actions taken to prepare the unit for follow-on tasks. As with consolidate, the platoon leader and platoon subordinate leaders plan and prepare for reorganization during TLP. During reorganization, these small-unit leader ensures the following actions are taken:

Provide essential medical treatment and evacuate casualties, as necessary.
Treat and evacuate wounded detainees and process the remainder of detainees.
Reestablish key leaders using the designated succession of command.
Reestablish manning of key weapons and equipment: CCMS, machine guns, grenade launchers, radio telephone operators.
Reestablish communications within the organization to higher and to adjacent units.
Cross-level personnel and adjust forces as required to support operations.
Conducts resupply operations.
Redistribute ammunition.
Conduct required maintenance.
Continue improving defensive positions, as necessary.

5-170. At the conclusion of an engagement, the platoon may continue the defense, or if ordered, transition to the offense. The platoon leader considers the higher commander’s concept of operations, friendly capabilities, and enemy situation when making this decision. All missions should include plans for exploiting success or assuming a defensive posture.

5-171. A defending unit may transition from defensive operations to the retrograde as a part of continuing operations. A retrograde (see paragraph 5-174) usually involves a combination of a delay, withdrawal, and retirement that may occur simultaneously or sequentially. As in other missions, the leader’s concept of operations and intent drive planning for the retrograde. Each form of retrograde has its unique planning considerations, but considerations common to all retrogrades are risk, the need for synchronization and security.

5-172. A company commander may order a defending platoon to conduct a hasty operation (for example, counterattack) or participate in a movement to contact. As part of a reserve force, the platoon and squad may execute a counterattack to destroy exposed enemy elements and free decisively engaged friendly elements. A base-of-fire element suppresses or fixes the enemy force while the counterattack element moves on a concealed route to firing positions from which it can engage the enemy in the flank and rear. The counterattack force must maneuver rapidly to its firing position, often fighting through enemy flank security elements, to complete the counterattack before the enemy can bring follow-on forces forward to influence the fight.

5-173. Execution of the counterattack is an attack by part or all of a defending force against an enemy attacking force, for such specific purposes as regaining ground lost or cutting off or destroying enemy advance units, and with the general objective of denying to the enemy the attainment of the enemy’s purpose in attacking. Planning and preparation considerations for counterattack vary depending on the purpose and location of the operation. For example, the counterattack may be conducted forward of friendly positions, requiring a force not in contact or the reserve force to move around friendly elements and through their protective and tactical obstacles. In other situations, the platoon leader may use a counterattack to block, fix, or contain a penetration within its AO. In any case, the counterattacking force conducts the counterattack as an enemy-oriented task.

Note. The mobile defense is a type of defensive operation that concentrates on the destruction or defeat of the enemy through a decisive attack by a striking force (ADP 3-90). Mobile defenses focus on defeating or destroying the enemy by allowing enemy forces to advance to a point where they are exposed to a decisive counterattack by the striking force—a dedicated counterattack force in a mobile defense constituted with the bulk of available combat power (ADP 3-90). The fixing force—a force designated to supplement the striking force by preventing the enemy from moving from a specific area for a specific time (ADP 3-90)—holds attacking enemy in position, to help channel attacking enemy forces into ambush areas, to retain areas from which to launch the striking force.
Mobile defenses require an AO of considerable depth. Commanders shape their battlefields causing enemy forces to overextend their lines of communications, expose their flanks, and dissipate their combat power. Commanders move friendly forces around and behind enemy forces to cut off and destroy them. Divisions or larger formations usually conduct a mobile defense because of their ability to fight multiple engagements throughout the width, depth, and height of their AO, while simultaneously resourcing the striking, fixing, and reserve forces. Typically, the striking force in a mobile defense consists of one-half to two-thirds of the defender’s combat power. The striking force decisively engages the enemy as it becomes exposed in attempts to overcome the fixing force.
The Infantry rifle platoon’s missions as part of a larger force in a mobile defense are similar to the missions in area defense. The platoon is generally part of the fixing force. As part of the fixing force, the platoon defends within its assigned AO, although the AO might be larger than usual. Weapons system from within the Infantry weapons company of the Infantry battalion may be attached to the platoon to enable better coverage of the larger AO and to engage enemy armor heavy forces from fixed positions within the area defense. (See ATP 3-21.10 for additional information.)

Next Page : Section 4 – Retrograde