text: Chapter 9
  points, gaps, or lanes along the FEBA as the security forces displaces. This usually occurs in one location at
  a time until all security forces complete their retrograde. However, the security force may pass in sequence
  based on enemy pressure. Transfer of responsibility occurs forward of the FEBA at the BHL. Security forces
  make any required last-minute coordination with MBA forces at contact points to ensure their rapid passage
  through the MBA. Figure 9-7 illustrates the use of a BHL and other control measures associated with the
  rearward movement of forward security forces.
  VARIATIONS OF THE AREA DEFENSE
  9-54. The variations of the area defense have unique conditions and their own planning considerations. The
  three variations of area defense are:
  * Defense of a linear obstacle.
  * Perimeter defense.
  * Reverse slope defense.
  9-55. When determining their variation of the area defense, units decide the location of defensible terrain
  within their assigned areas based on terrain characteristics and their estimate of the enemy force’s chosen
  COA. Those terrain characteristics include terrain relief patterns, avenues of approach into and within the
  assigned areas, the location of any key or decisive terrain, and existing obstacles and choke points, including
  rivers and fording sites. The other mission variables also influence this decision.
  DEFENSE OF A LINEAR OBSTACLE
  9-56. Units may conduct a variation of an area defense along or behind a linear obstacle (see figure 9-7).
  Linear obstacles, such as mountain ranges or rivers generally favor a forward defense. The planning,
  preparation, and execution of a defense of a linear obstacle is not different than any other area defense. The
  main difference is that the defense focuses on the linear natural or man-made obstacle as opposed to a separate
  key terrain feature, such as an urban area or hill.
  9-57. A defending force seeks to defeat any enemy forces attempting to seize a bridgehead across a linear
  obstacle. Local defending units immediately counterattack any enemy bridgeheads to destroy enemy forces
  located within the bridgehead, while higher echelons attempt to isolate enemy bridgehead sites. If an enemy
  force seizes a bridgehead and strikes out rapidly, it could quickly penetrate the defending force. An enemy
  penetration requires units to conduct either a counterattack, delay, or a withdrawal.
  9-58. It is extremely difficult to deploy in strength along the entire length of a linear obstacle. Defending
  units must conduct economy of force measures in some areas. In an area defense, units’ use of a defense in
  depth accepts the possibility that enemy forces may conduct a crossing at a given point. The depth of the
  defense should prevent an enemy force from rapidly exploiting its success. It also diffuses the enemy force’s
  combat power by causing enemy forces to contain bypassed friendly defensive positions in addition to
  continuing to attack friendly positions in greater depth. Once the enemy force secures several bridgeheads,
  the defending force moves to contain them. Defending commanders may choose not to counterattack until
  they can mass overwhelming combat power. They will probably choose to eliminate the bridgeheads
  sequentially in this case. However, defenders risk allowing enemy forces to establish and fortify bridgeheads
  and crossing sites to the extent that a counterattack force is unable to eliminate them.