text: Chapter 9
  Use of Terrain
  9-75. Proper evaluation and organization of an area’s terrain are essential to maximize the effectiveness of a
  force conducting perimeter defense. Units consider—
  * Natural defensive characteristics of the terrain.
  * Using artificial obstacles to enhance the natural defensive characteristics of the terrain.
  * Existing roads, railways, and waterways used for military LOCs and civilian commerce.
  * Controlled land areas surrounding the perimeter to a range beyond that of enemy mortars and
  rockets.
  Security
  9-76. An effective security requires early warnings. Early warnings of pending enemy actions allow units
  the time to react to any threat. Combat outposts, patrols, sensors, target acquisition radars, and aerial
  surveillance provide early warning. Civil information, human intelligence, and observed changes in a
  populations pattern of life near the position are excellent indicators of pending enemy actions. Security
  measures vary with the enemy threat, forces available, and the other mission variables; however, all around
  security is essential.
  Mutual Support
  9-77. Units position defending forces to ensure mutual employment of defensive resources, such as crew-
  served weapons, observation, and maneuver elements. Mutual support between defensive elements requires
  careful planning, positioning, and coordination because of the circular aspects of the perimeter defense. They
  use surveillance, obstacles, prearranged indirect fires, and maneuver elements to exploit or reinforce fires to
  control any gaps in the perimeter. Defensive plans provide for using all available support, including indirect
  fire, attack helicopters, and fixed-wing aircraft.
  All-Around Defense
  9-78. In defensive planning, units prepare to defend against an enemy attack from any direction and designate
  the unit defending the most likely enemy avenue of approach the main effort. They employ flexible plans
  and position their reserve to react to any threat. They commit maneuver elements and supporting weapons to
  detect, engage, and destroy an attacking enemy force. Commanders assign defensive positions and sectors of
  fire to all personnel within the perimeter.
  Defense in Depth
  9-79. Alternate, subsequent, and supplementary positions, combat outposts, and mutually supporting strong
  points forward of the perimeter extend the depth of a defense. Units plan fires to the maximum range
  throughout a defensive area. They may place protective obstacles around critical locations within the
  perimeter during periods of reduced visibility to disrupt the enemy’s plan and add depth to the defense.
  Responsiveness
  9-80. An attacking enemy force has the advantage of deciding when, where, and with what force to attack.
  Defending units prepare plans—including counterattack plans—and rehearse, assess, and revise them as
  necessary. The defensive plan contains procedures for timely responses by fire support teams and maneuver
  forces.
  Maximum Use of Offensive Action
  9-81. Since the objective of a perimeter defense is to maintain a secure position, units use offensive actions
  to engage enemy forces outside the perimeter. On initial occupation of a perimeter, friendly forces take
  offensive actions to destroy enemy forces in the immediate area. Once the perimeter area is clear, a relatively
  smaller force can defend the perimeter, thereby releasing other forces for other operations. Units employ
  patrols, raids, ambushes, aerial attacks, and supporting fires to harass and destroy enemy forces to prevent
  them from threatening the perimeter. They maintain constant communications and a COP with subordinates