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Previous: 3-152: Preparations

3-153. 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 3-14, page 3-46.) It 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 of 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 no most likely enemy approach is identified, 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 3-15) 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.

3-154. The most difficult aspect of the Y-shape perimeter defense is the fire control measures required. To fight this defense without casualties from friendly fire, the leaders must ensure the limits of fire for each weapon do not allow fires into the adjacent squad 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 a final protective line 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, nonpersistent mines, or obstacles to reduce the need for direct fires in these areas.
• Identify those positions at most risk to friendly fires 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 platoon.
• Consider allowing the enemy to penetrate well into the engagement areas and destroy him 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, he may fix the platoon with direct fires and destroy it with massed indirect fires.

Next: 3-155: Reverse-Slope Defense

Go Back To: U.S. Army FM 3-21.8: The Infantry Rifle Platoon and Squad