text: Area Defense
  within the perimeter. Commanders direct their subordinates to conduct appropriate actions to remove threats
  located within their assigned areas and sectors of fire.
  Executing a Perimeter Defense
  9-82. Attacks against a perimeter may range from long-range sniper, mortar, rocket fire, or attacks by suicide
  demolition squads to attacks by major enemy ground and air forces. Mortars, artillery, tanks, and anti-armor
  missile systems from within the perimeter engage enemy forces at long ranges. As an attack comes within
  small-arms range, other weapons on the perimeter engage enemy forces. If the assault continues, the
  defending force employs its FPFs. If enemy forces penetrate the perimeter, the reserve blocks the penetration
  or counterattacks to restore the perimeter. After committing the initial reserve, commanders reconstitute
  another reserve to meet other threats. This force normally comes from an unengaged unit on a different
  portion of the perimeter. If commanders use an unengaged force to constitute a new reserve, they must retain
  sufficient forces to defend the vacated sector, unless the situation forces commanders to assume that degree
  of risk.
  9-83. Sustainment elements may provide support from within the perimeter or from outside locations,
  depending on the mission and the status of the unit providing the defensive perimeter, type of transport
  available, weather, and terrain. Commanders often sustain units in contested areas without secure ground
  LOCs by air.
  Find the Enemy
  9-84. This typically occurs once friendly security forces make initial enemy contact. The security force does
  not idly await the arrival of the enemy. They take an active role as part of the information collection activities
  to find the enemy and understand their location, disposition, strength, and course of action.
  9-85. The security force also conducts counterreconnaissance against enemy reconnaissance and intelligence
  assets. Destroying these assets allows units to force enemy units to deploy into formations on the ground of
  those commanders’ choosing, thus helping to disrupt and desynchronize the enemy force’s plan. This also
  prevents the enemy from understanding the composition and disposition of the main effort of the defense and
  forces the enemy to attack into an unknown situation.
  Fix the Enemy
  9-86. Units do everything possible to limit an enemy force’s options when conducting an area defense. They
  conduct supporting efforts to constrain the enemy into specific COAs, control enemy movements, or fix
  enemy forces in given locations. Units continue to find, delay, or attrit enemy forces for follow on and reserve
  to keep them from entering the MBA when executing these operations.
  9-87. Units have several options to help fix attacking enemy forces. They can design supporting efforts—
  such as securing the flanks and point of a penetration—to fix enemy forces and allow friendly forces to
  execute decisive maneuver elsewhere. Combat outposts and strong points can also deny enemy force’s
  movement to or through a location.
  9-88. Units use obstacles to fix, turn, block, or disrupt to limit the enemy force’s options. Properly executed
  obstacles result from top-down and bottom-up obstacle planning and emplacement. Blocking forces can also
  affect enemy movement. A blocking force may achieve its mission from a variety of positions depending on
  the mission variables.
  Finish the Enemy
  9-89. The main effort occurs in the MBA in an area defense. This is where the effects of supporting efforts,
  coupled with sustaining operations, combine with the main effort of the main body forces to defeat enemy
  forces. Commanders’ goals are to prevent enemy forces from further advances by using a combination of
  fires from prepared positions, obstacles, and mobile reserves.
  9-90. Generating massed effects is especially critical to units conducting the defense of large areas against
  enemy forces with significant advantages in combat power. An attacking enemy force has the ability to select
  the point and time of attack. Therefore, an attacking enemy force can mass forces at specific points, thus