ADP 3-90, page 71

The Defense 31 July 2019 ADP 3-90 4-11 Ensure Mutual Support 4-61. Mutual support exists when positions and units support each other by direct and indirect fires, thus preventing an enemy force from attacking one position without being subjected to fire from one or more adjacent positions. Mutual support increases the strength of all defensive positions, prevents defeat in detail, and helps prevent enemy infiltration between positions. Battle positions achieve the maximum degree of mutual support when they are located to observe or monitor the ground between positions or when they conduct patrols to prevent any enemy infiltration. At night or during periods of limited visibility, commanders may position small units closer together to retain the advantages of mutual support. Unit leaders must coordinate the nature and extent of their mutual support. Mobility 4-62. During a defense, mobility tasks include breaching obstacles, clearing routes, and constructing and maintaining combat roads and trails to support counterattacks. Mobility tasks are those combined arms activities that mitigate the effects of obstacles to enable freedom of movement and maneuver. Engineer units usually perform these tasks. Enemy fires and friendly use accelerate the normal wear on routes. Engineers maintain the trafficability of those routes. Enemy fires may necessitate deploying engineer equipment, such as assault bridging and bulldozers, forward. During a counterattack, engineer breaching systems open closed lanes or breach hasty minefields placed by a retrograding enemy force. 4-63. Commanders establish the priority of mobility support based on the mission variables. This support consists mainly of reducing obstacles and improving or constructing combat roads and trails to allow tactical support vehicles to accompany combat vehicles. Commanders coordinate carefully to ensure that units leave lanes or gaps in their obstacles that allow for the repositioning of main body units and the commitment of the counterattacking force. CBRN reconnaissance systems also contribute to a force’s mobility in a contaminated environment by marking contaminated and clean routes as well as providing CBRN expertise while developing alternate COAs. Countermobility 4-64. Commanders designate obstacle zones, belts, and groups depending on their authority. When planning obstacles, commanders and staffs also consider future operations. Commanders design obstacles for current operations so they do not hinder planned future operations. Any commander authorized to employ obstacles can designate certain obstacles to shape the battlefield. There are two categories of reinforcing obstacles: tactical and protective. Tactical obstacles shape enemy maneuver to maximize the effects of fires. Tactical obstacles deny the ability of a force to move, mass, and reinforce; therefore, they affect the tempo of operations. There are three types of tactical obstacles: directed, situational, and reserved. Commanders employ protective obstacles to protect people, equipment, supplies, and facilities against threats. Protective obstacles have two roles, defense or security. 4-65. Obstacles can provide additional protection from enemy attacks by forcing an enemy force to spend time and resources to breach or bypass them. Effective obstacles block, turn, fix, or disrupt, forcing an enemy to attempt to breach or bypass them. A commander integrates reinforcing obstacles with existing obstacles to halt or slow enemy movement, canalize enemy movement into engagement areas, and protect friendly positions and maneuver. The primary purpose for integrating obstacles with fires is to enhance the effectiveness of those fires. Obstacles that are not covered by fire generally disrupt only the leading elements of an attacking force for a short time. 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. 4-66. Commanders designate the unit responsible for creating and overwatching each obstacle. Commanders may retain execution authority for some obstacles—called reserved obstacles—or restrict the use of some types of obstacles to allow other battlefield activities to occur. All units must know which gaps or lanes— through obstacles and crossing sites—to keep open for movements, as well as the firing and self-destruct times of scatterable mines to prevent delays in movement. Commanders must be specific and clear in their orders for executing reserved obstacles and closing lanes. As each lane closes, the closing unit reports the lane’s closure to prevent displacing units from moving into active obstacle areas.