text: The Defense
  * Facilitating the disengagement of ground forces.
  * Countering enemy activities in the echelon support area, in particular enemy airborne or air assault
  forces.
  * Resupplying defending forces with Class IV barrier material or facilitating casualty evacuation.
  * Assisting in the countermobility effort.
  These tasks may require additional ground forces to achieve the desired effect.
  Ensure Mutual Support
  8-66. Mutual support exists when positions and units reinforce each other with direct and indirect fires, thus
  preventing the enemy from attacking one position without one or more adjacent positions subjecting it to
  fires. Mutual support increases the strength of all defensive positions, prevents defeat in detail, and helps
  prevent infiltration between positions. Tactical positions achieve maximum degree of mutual support when
  they are located to observe or monitor the ground between their positions or conduct patrols to prevent any
  enemy infiltration. During periods of limited visibility, units position small tactical units closer together to
  retain the advantages of mutual support. Unit leaders coordinate the nature and extent of their mutual support.
  Countermobility
  8-67. In the defense, units normally prioritize engineer efforts on countermobility and survivability over
  mobility. This means that defending units typically require large quantities of Class IV and Class V material
  and specialized equipment to construct obstacles and fighting and survivability positions. Units ensure that
  they synchronize these efforts with their sustainment plans.
  8-68. Units plan to canalize enemy forces into a salient. In this case, they take advantage of the enemy force’s
  forward orientation by fixing the enemy force and then delivering a blow to the enemy force’s flank or rear.
  As an enemy’s attacking force culminates and assumes a defensive posture, friendly defending units rapidly
  coordinate and concentrate all fires against unprepared and unsupported segments of the attacking enemy
  force. Friendly forces may deliver these fires simultaneously or sequentially.
  8-69. When planning obstacles, units consider not only current operations but also future operations. They
  design obstacles for current operations, so they do not hinder future operations. Units can designate certain
  obstacles to shape the battlefield as high priority reserved obstacles. Units assign responsibility for
  preparation of reserved obstacles to a subordinate unit but retain authority for ordering their completion. One
  example of a reserved obstacle is a destruction of a highway bridge over a major river. Such obstacles receive
  the highest priority in preparation and, if ordered, execution by the designated subordinate unit.
  8-70. Units integrate reinforcing obstacles with existing obstacles to improve the natural restrictive nature
  of the terrain to halt or slow enemy movement, canalize enemy movement into engagement areas, and protect
  friendly positions and maneuver. They must integrate their obstacles with fires to be effective. This requires
  the ability to deliver effective fires well beyond the obstacle’s location. When possible, units conceal
  obstacles from hostile observation. They coordinate obstacle plans with adjacent units and conform to the
  obstacle zone or belts of higher echelons.
  8-71. Effective obstacles block, turn, fix, disrupt, or force the enemy to attempt to breach them. A defending
  unit tries to predict enemy points of breach based on terrain and probable enemy objectives. A defending
  force develops means to counter enemy breach attempts, such as pre-coordinated fires. The attacker will try
  to conceal the time and location of the breach. The defending unit’s plan addresses how to counter such a
  breach, to include reestablishing the obstacle by using scatterable mines and other techniques.
  8-72. Improvement to the defense is continuous. Given time and resources, a defending force constructs
  additional obstacle systems in depth, paying special attention to its assailable flanks and rear. The rear is
  especially vulnerable if noncontiguous assigned areas or nontraditional threats exist. Obstacle systems can
  provide additional protection from enemy attacks by forcing an enemy to spend time and resources to breach
  or bypass them. This gives the defending force more time to engage enemy forces attempting to execute
  breach or bypass operations.
  8-73. Units designate the subordinate unit responsible for establishing and securing each obstacle. They may
  retain execution authority for some obstacles or restrict the use of some types of obstacles to allow other