text: Area Defense
  Figure 9-7. Example of a defense of a linear obstacle
  PERIMETER DEFENSE
  9-59. Units may employ the perimeter defense as a variation when conducting an area defense when the unit
  is required to orient on all directions. They also use it in other circumstances, such as when 360-degree
  security is required when taking a temporary halt during a troop movement.
  9-60. The prerequisites for a successful perimeter defense are aggressive patrolling and security operations
  outside the perimeter. A unit within the perimeter can perform these activities, or another force, such as the
  territorial defense forces of a local nation in which hostilities occur, can perform them. A unit can organize
  a perimeter defense to accomplish a specific mission, such as protecting a firebase or providing immediate
  self-protection, such as during resupply operations when all around security is required. Commanders
  establish a perimeter when a unit must hold key terrain, such as a strong point, or when it must defend itself
  in noncontiguous areas where a unit cannot tie its defense in with adjacent units. This occurs when a unit is
  operating behind enemy lines or when it is securing an isolated objective, such as a bridge, mountain pass,
  or airfield. Bypassed or isolated friendly forces also need to form a perimeter to allow them to defend in
  place. Forces located within the confines of a base or base cluster in their echelon support also need to
  establish a perimeter defense. Figure 9-8 on page 9-14 depicts an example of a perimeter defense.