--Perimeter Defense

3-151. The platoon leader can employ the perimeter defense as an option when conducting an area or mobile defense. A perimeter defense is a defense oriented in all directions. (See figure 3-13.) The Infantry 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 secure itself against terrorist or insurgent attacks in an urban area.
  • This technique also may apply if the platoon must conserve or build combat power in order to execute offensive tasks or patrolling missions.
  • When it must hold critical terrain in areas where the 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 conducts occupation of an independent assembly area or reserve position.
  • When it begins preparation of a strongpoint.
  • When it is directed to concentrate fires into two or more adjacent avenues of approach.
Platoon perimeter defense

Figure 3-13. Platoon perimeter defense

Soldiers establish a security perimeter.

Soldiers establish a security perimeter.


3-152. The Infantry platoon prepares a perimeter defense when there are no friendly units adjacent to it. A perimeter defense might be used in a reserve position, in an AA or patrol base, on a follow-on decentralized platoon operation during resupply or when the platoon is isolated. The following actions constitute setting up a perimeter defense ─

  • 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 platoon must be prepared to defend in all directions.
  • The platoon leader assigns squads to cover the most likely approach, and prepares alternate and supplementary positions within the perimeter.
  • Javelins cover likely armor approaches.
  • They 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 or designated marksman should cover likely or suspected enemy positions or observation posts.
  • Snipers and 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.
  • They should dig in and have covered ammunition storage bunkers.
  • If possible, hold one or more rifle team 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-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.


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.) 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.
Y-shaped perimeter defense

Figure 3-14. Y-shaped perimeter defense

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.
Modified Y-shape perimeter defense

Figure 3-15. Modified Y-shape perimeter defense

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, non-persistent 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.