text: The Defense
  8-56. Units conduct survivability moves among their primary, alternate, supplementary, and subsequent
  positions based on the mission variables. A survivability move is a move that involves rapidly displacing a
  unit, command post, or facility in response to direct and indirect fires, the approach of a threat or as a
  proactive measure based on intelligence, meteorological data, and risk assessment of enemy capabilities and
  intentions (ADP 3-90).
  Maintain Security
  8-57. Units use security operations to prevent enemy forces from determining friendly locations, strengths,
  weaknesses, and to preserve combat power. They also try to delay and disrupt an attacking force by making
  it deploy prematurely. Defending units can mitigate an attacker’s inherent advantage of initiative regarding
  the time, place, plan, direction, strength, and composition of the attack by disrupting enemy formations and
  countering reconnaissance efforts, causing them to blindly attack into prepared defenses. Defending units
  counter enemy reconnaissance through both active and passive measures. They must not permit enemy
  information collection assets to determine the precise location and strength of defensive positions, obstacles,
  engagement areas, and reserves. First, the defending force conducts reconnaissance to gain and maintain
  contact with the enemy. Second, each echelon normally establishes a security area forward of its MBA. The
  security area is separated from the MBA by the FEBA. It extends as far to the front and flanks as necessary
  for the security force to deploy. Forces in the security area furnish information on enemy forces and delay,
  deceive, and disrupt enemy forces and conduct counterreconnaissance. All units conduct local security
  operations within their assigned areas to prevent enemy forces from disrupting operations, to include echelon
  support areas. Units implement OPSEC and other protection measures to deny enemy forces information
  about friendly dispositions.
  Disrupt the Enemy Attack at Every Opportunity
  8-58. The defending force conducts operations throughout the depth of the enemy’s formation to destroy key
  enemy units and assets, particularly their artillery and reserves, or disrupt their timely introduction into battle
  at the point of engagement. This allows the defending force to regain the initiative. The defending force
  conducts spoiling attacks to disrupt the enemy force’s concentrations and attack preparations. The defending
  force counterattacks enemy successes rapidly with a friendly reserve, the forces at hand, or a striking force
  before enemy forces can exploit success. Units use direct fires to mass combat power. Units designate direct
  fire control measures to focus, shift, or distribute direct fires on enemy formations or within engagement
  areas.
  Mass the Effects of Combat Power
  8-59. A defending force must mass its combat power to overwhelm enemy forces and regain the initiative.
  When required, units use economy of force measures in areas that do not involve the main effort to mass
  forces for use at the decisive point. In an area defense, defending units use engagement areas to concentrate
  combat power from mutually supporting positions. In a mobile defense, units use the striking force to
  generate combat power at decisive points. Another way forces can apply the effects of mass is by committing
  the reserve.
  8-60. Typically, defending units engage advancing enemy forces using two primary methods—echelonment
  of fires and simultaneous fires. In echelonment of fires, units engage an advancing enemy force at the
  maximum effective range of available weapon systems. They then employ an increasing volume of fire by
  engaging with shorter range systems as the attacking enemy force continues to close on their positions while
  continuing to engage the attacker with longer range systems. This allows for defending units to attrit and
  defeat enemy forces as far forward of friendly defensive positions as is possible. It also allows the defender
  to engage the enemy for longer periods which normally allows for more kills forward of the defender’s
  positions. Defending units normally employ this method of engagement against enemy formations of similar
  or larger size than defending forces. The major disadvantage of this method is that once defenders employ
  indirect fire systems, it allows enemy forces to locate the firing positions of those systems. This allows enemy
  forces to engage the defending forces with fires. Also at lower tactical levels, it may make flank shots against
  enemy armored systems more difficult to obtain at longer ranges. (See ATP 3-09.42 for more information on
  echelonment of fires.)