text: Chapter 4
  The primary focus for combat engineers in support of defensive operations is to enable combined arms
  obstacle integration (countermobility) to facilitate mobility for friendly repositioning or counterattacking
  forces. Defensive operations demand the greatest survivability effort. Activities in the defense include
  constructing survivability positions for headquarters, artillery, air and missile defense, and critical equipment
  and supplies. Activities also include preparing individual and crew-served fighting positions and defilade
  fighting positions for combat vehicles. The use of engineer work timelines is essential, and digging assets are
  intensively managed. During this period, countermobility efforts compete with survivability resources and
  assets. Because of this, it is critical that maneuver commanders provide clear guidance on resources and
  priorities of effort. General engineers support tasks that exceed the capability of the combat engineer force
  and provide more extensive support to the mobility of repositioning counterattack forces. Examples of
  expected missions include the—
  * Construction and integration of obstacles.
  * Preparation of fighting positions and survivability positions in depth.
  * Upgrade and repair of routes that facilitate the repositioning of forces throughout the AO.
  During preparation, engineer assets are postured with the task-organized gaining or supported
  headquarters, and they initiate the engineer work effort. The equipment work effort is a balance between
  countermobility and survivability, as determined by the commander. The effort continues throughout
  preparation activities until it is complete or until it is no longer feasible. Significant coordination is required
  to resource the materials required for constructing obstacles and fighting positions and to integrate the
  obstacles with friendly fire effects. Designated combat engineers provide mobility support for the reserves
  or mobile strike force. The engineer staff officer, at appropriate echelons, coordinates for engineer
  reconnaissance and surveillance assets to identify specific enemy engineering capabilities (breaching,
  bridging, and countermobility assets) to nominate those capabilities for targeting, ensuring timely
  destruction.
  At the theater level, general engineer support is continuously conducted to harden and prepare
  protective positions for facilities and installations. These activities are primarily applied through the
  protection warfighting function. General engineering support to protection and survivability continues
  throughout operations as improvements are continuously reassessed and an additional effort is made
  available. The theater may employ barriers in support of countermobility. (See JP 3-15 for a further
  discussion of barriers.) Other general engineer activities applied to enable the sustainment warfighting
  function may also be critical to the preparation and conduct of the defense. Enabling mobility throughout the
  depth of the AO will remain an engineer mission.
  BASICS OF THE DEFENSE
  The types of defensive operations are mobile defense, area defense, and retrograde. These types have
  significantly different concepts and must be dealt with differently during planning and execution as follows:
  * Mobile defense. Engineer support to a mobile defense focuses on using obstacles to defeat enemy
  maneuver and on providing mobility to the striking force and reserves. Countermobility and
  survivability assets support the fixing force, while many mobility assets support the striking force.
  Obstacle control coordinated at the division echelon is directed at the most likely enemy COA
  rather than the terrain and may be restricted to assure striking force mobility. Situational obstacles
  are advantageous in the mobile defense. These obstacles allow the commander to exploit enemy
  vulnerabilities, exploit success, separate follow-on forces, and provide flank protection.
  * Area defense. In an area defense, the focus of engineer effort is on providing the maneuver
  commander with the ability to hold terrain while enabling maneuver units to concentrate fires from
  static positions. Engineers help identify key and decisive terrain that supports the commander’s
  concept of operations, with a focus on where the commander wants to kill the enemy. During
  obstacle planning, obstacle control measures are designed to give maximum flexibility to
  subordinate units while focusing the tactical obstacle effort on terrain retention. The engineer staff
  officer must advise the commander of the resource requirements of each subordinate unit based
  on its assigned essential M/CM/S and other engineering tasks. The division echelon must balance
  these engineer resource requirements. Planning for scatterable mines enables commanders to
  accept less risk by not allowing the enemy to cross terrain-shaping obstacles.