text: Area Defense
  division conducting a defense in depth with subordinate BCTs deployed in noncontiguous assigned areas
  with enemy avenues of approach shown.) Commanders usually decide to conduct a defense in depth when—
  * Their defensive missions are not restrictive and allow them to operate throughout the depths of
  their assigned areas.
  * The terrain within their assigned areas does not favor a forward defense, and there is better
  defensible terrain deeper within their assigned areas.
  * There is significant depth available, allowing the trading of space for time.
  * The cover and concealment on or near the FEBA is limited.
  Figure 9-4. Example defense in depth
  9-33. Large units, such as divisions or corps, employing a defense in depth can conduct an area defense on
  a wider frontage than they can if they adopt a forward defense. This is because a forward defense has no time
  or maneuver space to reposition forces. A defense in depth allows units to use security forces in the forward
  part of their MBA to identify the enemy force’s main effort and control the depth of the enemy force’s
  penetration into their MBA. By their defensive actions, these forces provide units with time to react to enemy
  actions and allow defending units to take offensive steps to eliminate enemy options, such as conducting
  counterattacks into the flanks of advancing enemy forces.
  Forward Defense
  9-34. When units defend forward within their assigned areas, they organize their forces so they can commit
  most of their available combat power early in their defensive efforts. To accomplish this, units may deploy
  their forces forward in their respective assigned areas or plan counterattacks well forward in the MBA or
  even beyond the MBA. Units can conduct their main effort from forward defensive positions near the FEBA
  in a forward defense. Figure 9-5 on page 9-8 is a sketch illustrating this concept. Units concentrate significant
  portions of their available combat power toward engagement areas located along their FEBA. The intent is
  to prevent significant enemy penetrations into their defensive areas. Units conduct forward defense
  operations to retain these positions along their FEBA and counterattack any enemy penetrations. However,
  the defending force’s lack of depth may allow enemy forces to exploit success if they penetrate the friendly
  force’s main defensive positions.