text: FM 7-100.1________________________________________________________________________________
  SITUATIONAL DEFENSE
  4-27. The OPFOR recognizes that the modern battlefield is chaotic, and fleet-
  ing opportunities to attack an enemy weakness will continually present
  themselves and just as quickly disappear. If the OPFOR determines that, by
  conserving resources in one area, it may be able to take advantage of a win-
  dow of opportunity in another, it may assume a situational (or hasty) defense.
  It may also do so when an OPFOR attack culminates prior to achieving the
  objective.
  4-28. The OPFOR may also be forced to employ a situational defense when it
  has been conducting offensive operations against a regional neighbor and in-
  tervention by a powerful extraregional force materializes more quickly than
  anticipated. Thus, the OPFOR may have to make the transition from regional
  to adaptive operations more rapidly than planned. Units may still be able to
  move into preplanned positions in complex terrain, but without some meas-
  ures they anticipated being able to take during transition operations. They
  may or may not have fully-prepared, complex battle positions, with engineer
  preparation, C3D measures, and logistics caches.
  4-29. The commander develops his assessment of the conditions leading to a
  situational defense rapidly and without a great deal of staff involvement. He
  provides a basic course of action to the staff, who then quickly turn that
  course of action into an executable operational directive.
  4-30. Organization of the battlefield in a situational defense is normally lim-
  ited to minor changes to existing control measures. Organization of forces in
  a situational defense typically relies on minor modifications to existing
  structure.
  4-31. The following are examples of conditions that might lead to a
  situational defense:
  • The enemy gains or regains air superiority sooner than anticipated.
  • An enemy counterattack was not effectively fixed.
  • An attacking force makes contact with an enemy formation it did not
  expect.
  ORGANIZING THE BATTLEFIELD FOR THE DEFENSE
  4-32. In his operation plan, the commander specifies the organization of the
  battlefield from the perspective of his level of command. Within his unit’s
  area of responsibility (AOR), as defined by the next-higher commander, he
  designates AORs for his subordinates, along with zones related to his own
  overall mission.
  4-33. In organizing the defensive battlefield, the operational commander
  organizes forces to begin attack of the combat system of the enemy force as
  soon as feasible. By attacking subsystems or components of the enemy’s
  combat system appropriate to the situation, the operational commander can
  create windows of opportunity for offensive action.