Infantry Drills

FM 3-21.8 – Chapter 8 – Section VI – Defensive Techniques

Section VI — Defensive Techniques

8-109.  The platoon will normally defend IAW command orders using one of these basic techniques:

– Defend an area.

– Defend a battle position.

– Defend a strongpoint.

– Defend a perimeter.

– Defend a reverse slope.


8-110.  Defending an area  sector allows a unit to maintain flank contact and security while ensuring unity of effort in the scheme of maneuver. Areas afford depth in the platoon defense. They allow the platoon to achieve the platoon leader’s desired end state while facilitating clearance of fires at the appropriate level of responsibility. The company commander normally orders a platoon to defend an area (Figure 8-8) when flexibility is desired, when retention of specific terrain features is not necessary, or when the unit cannot concentrate fires because of any of the following factors:

– Extended frontages.

– Intervening, or cross-compartmented, terrain features.

– Multiple avenues of approach.

8-111.  The platoon is assigned an area defense mission to prevent a specific amount of enemy forces from penetrating the area of operations. To maintain the integrity of the area defense, the platoon must remain tied to adjacent units on the flanks. The platoon may be directed to conduct the defense in one of two ways.

8-112.  He may specify a series of subsequent defensive positions within the area from where the platoon will defend to ensure that the fires of two platoons can be massed.

8-113.  He may assign an area to the platoon. The platoon leader assumes responsibility for most tactical decisions and controlling maneuvers of his subordinate squads by assigning them a series of subsequent defensive positions. This is done IAW guidance from the company commander in the form of intent, specified tasks, and the concept of the operation. The company commander normally assigns an area to a platoon only when it is fighting in isolation.


Figure 8-8. Concept of the operation for defending an area.


8-114.  The company commander assigns the defensive technique of defending a battle position to his platoons when he wants to mass the fires of two or more platoons in a company engagement area, or to position a platoon to execute a counterattack. A unit defends from a battle position to—

– Destroy an enemy force in the engagement area.

– Block an enemy avenue of approach.

– Control key or decisive terrain.

– Fix the enemy force to allow another friendly unit to maneuver.

8-115.  The company commander designates engagement areas to allow each platoon to concentrate its fires or to place it in an advantageous position for the counterattack. Battle positions are developed in such a manner to provide the platoon the ability to place direct fire throughout the engagement area. The size of the platoon battle position can vary, but it should provide enough depth and maneuver space for subordinate squads to maneuver into alternate or supplementary positions and to counterattack. The battle position is a general position on the ground. The platoon leader places his squads on the most favorable terrain in the battle position based on the higher unit mission and commander’s intent. The platoon then fights to retain the position unless ordered by the company commander to counterattack or displace. The following are basic methods of employing a platoon in a battle position:

– Same battle position, same avenue of approach.

– Same battle position, multiple avenues of approach.

– Different battle positions, same avenue of approach.

– Different battle positions, multiple avenues of approach.

Same Battle Position, Same Avenue of Approach

8-116.  Rifle squads are on the same battle position covering the same avenue of approach (Figure 8-9). The platoon can defend against mounted and dismounted attacks and move rapidly to another position.

8-117.  All squads are in the same battle position when the terrain provides good observation, fields of fire, and cover and concealment.

8-118.  Employing all the squads of the platoon on the same battle position covering the same avenue of approach is the most conservative use of the platoon. Its primary advantages are that it facilitates command and control functions because of the proximity of squad elements on the same approach and it provides increased security.


Figure 8-9. Same battle position, same avenue of approach.

Same Battle Position, Multiple Avenues of Approach

8-119.  Rifle squads occupy the same battle position but cover multiple enemy avenues of approach (Figure 8-10).


Figure 8-10. Same battle position, multiple avenues of approach.

Different Battle Positions, Same Avenue of Approach

8-120.  Rifle squads are on different battle positions covering the same avenue of approach (Figure 8-11). If positioned on separate battle positions, rifle squads must fight in relation to each other when covering the same avenues of approach. A weapons squad can provide supporting fires for the rifle squads from their primary, alternate, or supplementary positions. All squads are positioned to engage enemy forces on the same avenue of approach, but at different ranges.


Figure 8-11. Different battle positions, same avenue of approach.

Different Battle Positions, Multiple Avenues of Approach

8-121.     Squads may be employed on different battle positions and multiple avenues of approach (Figure 8-12) to ensure that the squad battle positions cannot be fixed, isolated, or defeated by the enemy.


Figure 8-12. Different battle positions, multiple avenues of approach.


8-122.  Defending a strongpoint (Figure 8-13) is not a common mission for an Infantry platoon. A strongpoint defense requires extensive engineer support (expertise, materials, and equipment), and takes a long time to complete. When the platoon is directed to defend a strongpoint, it must retain the position until ordered to withdraw. The success of the strong-point defense depends on how well the position is tied into the existing terrain. This defense is most effective when it is employed in terrain that provides cover and concealment to both the strongpoint and its supporting obstacles. Mountainous, forested, or urban terrain can be adapted easily to a strongpoint defense. Strongpoints placed in more open terrain require the use of reverse slopes or of extensive camouflage and deception efforts. This defensive mission may require the platoon to—

– Hold key or decisive terrain critical to the company or battalion scheme of maneuver.

– Provide a pivot to maneuver friendly forces.

– Block an avenue of approach.

– Canalize the enemy into one or more engagement areas.

Characteristics of the Strongpoint Defense

8-123.  The prime characteristic of an effective strongpoint is that it cannot be easily overrun or bypassed. It must be positioned and constructed so the enemy knows he can reduce it only at the risk of heavy casualties and significant loss of materiel. He must be forced to employ massive artillery concentrations and dismounted Infantry assaults in his attack, so the strongpoint must be tied in with existing obstacles and positioned to afford 360-degree security in observation and fighting positions.

Techniques and Considerations

8-124.  A variety of techniques and considerations are involved in establishing and executing the strongpoint defense, including considerations for displacement and withdrawal from the strongpoint.

8-125.  The platoon leader begins by determining the projected size of the strongpoint. He does this through assessing the number of weapons systems and individual Soldiers available to conduct the assigned mission, and by assessing the terrain on which the platoon will fight. He must remember that although a strongpoint is usually tied into a company defense and flanked by other defensive positions, it must afford 360-degree observation and firing capability.

8-126.  The platoon leader must ensure that the layout and organization of the strongpoint maximizes the capabilities of the platoon’s personnel strength and weapons systems without sacrificing the security of the position. Platoon options range from positioning CCMS outside the strongpoint (with the rifle squads occupying fighting positions inside it), to placing all assets within the position. From the standpoint of planning and terrain management, placing everything in the strongpoint is the most difficult option and potentially the most dangerous because of the danger of enemy encirclement.


Figure 8-13. Defending a strongpoint.

8-127.  In laying out the strongpoint, the platoon leader designates weapon positions that support the company defensive plan. Once these primary positions have been identified, he continues around the strongpoint, siting weapons on other possible enemy avenues of approach and engagement areas until he has the ability to orient effectively in any direction. The fighting positions facing the company engagement area may be along one line of defense or staggered in depth along multiple lines of defense (if the terrain supports positions in depth).

8-128.  The platoon’s reserve may be comprised of a fire team, squad, or combination of the two. The platoon leader must know how to influence the strongpoint battle by employing his reserve. He has several employment options including reinforcing a portion of the defensive line or counterattacking along a portion of the perimeter against an identified enemy main effort.

8-129.  The platoon leader should identify routes or axes that will allow the reserve to move to any area of the strongpoint. He should then designate positions the reserve can occupy once they arrive. These routes and positions should afford sufficient cover to allow the reserve to reach its destination without enemy interdiction. The platoon leader should give special consideration to developing a direct fire plan for each contingency involving the reserve. The key area of focus may be a plan for isolating an enemy penetration of the perimeter. Rehearsals cover actions the platoon takes if it has to fall back to a second defensive perimeter, including direct fire control measures necessary to accomplish the maneuver. FPF may be employed to assist in the displacement.

8-130.  Engineers support strongpoint defense by reinforcing the existing obstacles. Priorities of work will vary depending on the factors of METT-TC, especially the enemy situation and time available. For example, the first 12 hours of the strongpoint construction effort may be critical for emplacing countermobility obstacles and survivability positions, and command and control bunkers. If the focus of engineer support is to make the terrain approaching the strongpoint impassable, the battalion engineer effort must be adjusted accordingly.

8-131.  The battalion obstacle plan provides the foundation for the company strongpoint obstacle plan. The commander or platoon leader determines how he can integrate protective obstacles (designed to defeat dismounted enemy Infantry assaults) into the overall countermobility plan. If adequate time and resources are available, he should plan to reinforce existing obstacles using field-expedient demolitions.

8-132.  Once the enemy has identified the strongpoint, he will mass all the fires he can spare against the position. To safeguard his rifle squads, the platoon leader must arrange for construction of overhead cover for individual fighting positions. If the strongpoint is in a more open position (such as on a reverse slope), he may also plan for interconnecting trenchlines. This will allow Soldiers to move between positions without exposure to direct and indirect fires. If time permits, these crawl trenches can be improved to fighting trenches or standard trenches.


8-133.  A perimeter defense allows the defending force to orient in all directions. In terms of weapons emplacement, direct and indirect fire integration, and reserve employment, a platoon leader conducting a perimeter defense should consider the same factors as for a strongpoint operation.

8-134.  The perimeter defense allows only limited maneuver and limited depth. Therefore, the platoon may be called on to execute a perimeter defense under the following conditions:

– Holding critical terrain in areas where the defense is not tied in with adjacent units.

– Defending in place when it has been bypassed and isolated by the enemy.

– Conducting occupation of an independent assembly area or reserve position.

– Preparing a strongpoint.

– Concentrating fires in two or more adjacent avenues of approach.

– Defending fire support or engineer assets.

– Occupying a patrol base.

8-135.  The major advantage of the perimeter defense (Figure 8-14) is the platoon’s ability to defend against an enemy avenue of approach. A perimeter defense differs from other defenses in that—

– The trace of the platoon is circular or triangular rather than linear.

– Unoccupied areas between squads are smaller.

– Flanks of squads are bent back to conform to the plan.

– The bulk of combat power is on the perimeter.

– The reserve is centrally located.


Figure 8-14. Perimeter defense with rifle team in reserve.

NOTE:  A variant of the perimeter defense is the use of the shaped defense, which allows two of the platoon’s squads to orient at any particular time on any of three engagement areas.


8-136.  The platoon leader’s analysis of the factors of METT-TC often leads him to employ his forces on the reverse slope (Figure 8-15). If the rifle squads are on a mounted avenue of approach, they must be concealed from enemy direct fire systems. This means rifle squads should be protected from enemy tanks and observed artillery fire.


Figure 8-15. Reverse-slope defense options.

8-137.  The majority of a rifle squad’s weapons are not effective beyond 600 meters. To reduce or prevent destruction from enemy direct and indirect fires beyond that range, a reverse-slope defense should be considered. Using this defense conflicts to some extent with the need for maximum observation forward to adjust fire on the enemy, and the need for long-range fields of fire for CCMS. In some cases it may be necessary for these weapons systems to be deployed forward while the rifle squads remain on the reverse slope. CCMS gunners withdraw from their forward positions as the battle closes. Their new positions should be selected to take advantage of their long-range fires, and to get enfilade shots from the depth and flanks of the reverse slope.

8-138.  The nature of the enemy may change at night, and the rifle squads may occupy the forward slope or crest to deny it to the enemy. In these circumstances, it is feasible for a rifle squad to have an alternate night position forward. The area forward of the topographical crest must be controlled by friendly forces through aggressive patrolling and both active and passive reconnaissance measures. The platoon should use all of its night vision devices to deny the enemy undetected entry into the platoon’s defensive area. CCMS are key parts of the platoon’s surveillance plan and should be positioned to take advantage of their thermal sights. The enemy must not be allowed to take advantage of reduced visibility to advance to a position of advantage without being taken under fire.

8-139.  The company commander normally makes the decision to position platoons on a reverse slope. He does so when—

– He wishes to surprise or deceive the enemy about the location of his defensive position.

– Forward slope positions might be made weak by direct enemy fire.

– Occupation of the forward slope is not essential to achieve depth and mutual support.

– Fields of fire on the reverse slope are better or at least sufficient to accomplish the mission.

– Forward slope positions are likely to be the target of concentrated enemy artillery fires.

8-140.  The following are advantages of a reverse-slope defense:

– Enemy observation of the position, including the use of surveillance devices and radar, is masked.

– Enemy cannot engage the position with direct fire without coming within range of the defender’s weapons.

– Enemy indirect fire will be less effective because of the lack of observation.

– Enemy may be deceived about the strength and location of positions.

– Defenders have more freedom of movement out of sight of the enemy.

8-141.  Disadvantages of a reverse-slope defense include the following:

– Observation to the front is limited.

– Fields of fire to the front are reduced.

– Enemy can begin his assault from a closer range.

8-142.  Obstacles are necessary in a reverse-slope defense. Because the enemy will be engaged at close range, obstacles should prevent the enemy from closing too quickly and overrunning the positions. Obstacles on the reverse slope can halt, disrupt, and expose enemy vehicles to flank antitank fires. Obstacles should also block the enemy to facilitate the platoon’s disengagement.

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