text: Area Defense
  location of the main defense. They position information collection assets and observation posts near or
  forward of the topographical crest to provide long-range observation of both the enemy force’s flanks and
  front. An observation post may vary in size from a two-man team to a rifle squad or to a multiple combat
  vehicle section in each position. Units employ sufficient forces to provide observation and a usually a screen
  for the MBA. During periods of reduced visibility, units should increase the numbers and sizes of these
  detachments to provide security against infiltration or unexpected attack. Aggressive night combat patrols
  and ambushes are an essential part of the security process.
  9-108. To achieve surprise and limit an enemy force’s ability to maneuver, units organize the main body
  into defensive positions to mass fires on attacking enemy forces as they cross the topographic crest. In a
  reverse slope defense, the key position denies enemy penetration and supports forward elements by fire. The
  defending force maintains observation and fires over the entire forward slope as long as possible to destroy
  enemy forces, thus preventing enemy forces from massing for a final assault. From defensive positions on
  the reverse slope, the close-in battle builds in intensity. The defending force does not fire its direct fire
  weapons, which are located throughout the MBA (on adjacent slope positions, counterslope positions, or
  reverse slope positions) until suitable targets appear. At the same time, the defending force shifts the effects
  of its indirect fires to those areas forward of the crest and forward military slope.
  9-109. When possible, other units on complementary terrain support units in reverse slope positions. This
  is especially desirable when those supporting units can observe and place fires on the crest and forward slope.
  In a defense on a counterslope (reverse forward slope), fires must cover the area immediately in front of the
  reverse slope positions to the topographical crest. Units organize defensive positions to permit fires on enemy
  approaches around and over the crest and on the forward slopes of adjacent terrain features, if applicable.
  The key factors that affect the organization of these areas are mutually supporting covered and concealed
  positions, numerous existing and reinforcing obstacles, the ability to bring devastating fires from all available
  weapons onto the crest, and a counterattack force. Depending on the terrain, the most desirable location for
  the reserve may be on the counterslope or the reverse military crest of the counterslope.
  Control Measures
  9-110. Defensive control measures introduced in previous chapters continue to apply. Units place
  engagement areas and obstacles on the reverse slope. The topographical crest normally marks the far edge of
  the engagement area. Defending forces must dominate the crest with fires to prevent enemy forces from
  successfully engaging the main body.
  Executing a Reverse Slope Defense
  9-111. When executing a reverse slope defense, units place special emphasis on—
  * The proper organization of the forward slope to provide observation across the entire front and
  security to the main BPs.
  * A fire support plan to prevent the enemy force’s occupation and use of the topographical crest.
  * A counterattack plan that specifies measures necessary to clear the crest or regain it from enemy
  control.
  * Fire support to destroy, disrupt, and attrit enemy forces on the forward slope.
  9-112. Units normally place FPFs along the topographical crest and employ them as enemy forces reach
  the first row of defiladed obstacles. Units use the reserve to counterattack and expel the enemy from the
  topographical crest if massed indirect fires do not defeat the attack.
  9-113. A reverse slope defense pursues offensive opportunities through surprise and deceptive actions. It is
  uniquely suited to infantry forces in mountainous terrain. When conducting a reverse slope defense, surprise
  results from defending in a manner for which the enemy forces are not prepared. Once this defense
  successfully halts an enemy attack, it may have limited further value because the effect of surprise is difficult
  to attain.
  9-114. The entire forward security force should not withdraw automatically as soon as the first enemy units
  reach the FEBA. Units can leave in place security elements located in areas where enemy forces have not
  advanced. These security forces adjust to the enemy force’s advance and continue to conduct security