Infantry Drills

A-42: Obstacles

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A-42. Leaders identify existing (inherent to terrain and either natural or man-made) and reinforcing (tactical or protective) obstacles limiting mobility in his area of operation. Reinforcing obstacles are constructed, emplaced, or detonated by military force:

  • Existing obstacles, natural include rivers; forests; mountains; ravines; gaps and ditches more than three meters wide; tree stumps and large rocks more than 18 inches high; forests with trees eight inches or more in diameter, with less than four meters between trees.
  • Existing obstacles, man-made include towns; canals; railroad embankments; buildings; power lines; telephone lines.
  • Reinforcing obstacles, tactical—tactical (reinforcing) obstacles inhibit the ability of the opposing force to move, mass, and reinforce. Examples include mine fields (conventional and situational); antitank ditches; wire obstacles.
  • Reinforcing obstacles, protective (reinforcing) obstacles offer close-in protection and are important to survivability.
  • Offensive considerations when analyzing obstacles and restricted terrain:
    • How is the enemy using obstacles and restricted terrain features?
    • What is the composition of the enemy’s reinforcing obstacles?
    • How will obstacles and terrain affect the movement or maneuver of the unit?
    • If necessary, how can I avoid such features?
    • How do I detect and, if desired, bypass the obstacles?
    • Where has the enemy positioned weapons to cover the obstacles, and what type of weapons is he using?
    • If I must support a breach, where is the expected breach site and where will the enemy be overwatching the obstacle?
    • How will the terrain affect the employment of mortars, medium machine guns, and Javelin missiles?
  • Defensive considerations when analyzing obstacles and restricted terrain:
    • Where does the enemy want to go? Where can I kill him? How do I get him to go there?
    • How will existing obstacles and restricted terrain affect the enemy?
    • How can I use these features to force the enemy into its engagement area, deny him an avenue, or disrupt his movement?
    • How will the terrain affect the employment of mortars, medium machine guns, and Javelin missiles?
  • Categories of terrain, unrestricted—terrain free of restrictions to movement, so no actions are needed to enhance mobility. For armored forces, unrestricted terrain typically is flat or moderately sloped, with scattered or widely spaced obstacles such as trees or rocks. This terrain generally allows wide maneuver and offers unlimited travel over well-developed road networks. It allows the platoon and squads to move with little hindrance.
  • Categories of terrain, restricted—terrain hindering movement somewhat. Little effort is needed to enhance mobility, but units might have to zigzag or make frequent detours. They could have a hard time maintaining optimum speed, moving in some types of combat formations, or transitioning from one formation to another. For armored forces, restricted terrain typically means moderate to steep slopes or moderate to dense spacing of obstacles such as trees, rocks, or buildings. Swamps and rugged ground are two examples of restricted terrain for Infantry forces. Poorly developed road systems may hamper logistical or rear area movement.
  • Categories of terrain, severely restricted—terrain which severely hinders or slows movement in combat formations unless some effort is made to enhance mobility. Engineer forces might be needed to improve mobility or platoon and squads might have to deviate from doctrinal tactics. For example, they might have to move in columns rather than in lines. Or, they might have to move much more slowly than they would like. For armored forces, steep slopes, densely spaced obstacles, and absence of a developed road system characterize severely restricted terrain.

Next: A-43: Avenues of Approach

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