text: Chapter 8
  battlefield activities to occur. Units allow subordinate units some flexibility in selecting the exact positioning
  of obstacles. However, 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. Units follow specific and clear orders for executing reserve obstacles and closing lanes. As
  each lane closes, the closing unit reports the lane’s closure to the higher echelon, subordinate, and adjacent
  headquarters to preclude displacing units from moving into unmarked or abandoned obstacles.
  8-74. Primarily company-sized and smaller units construct protective obstacles. Leaders within these small
  units ensure that observation and fires cover all obstacles to hinder breaching. Deliberate protective obstacles
  are common around fixed sites. Protective obstacles are a key component of survivability operations. They
  are tied in with final protective fires and provide friendly forces with close-in protection. Units at all echelons
  track defensive preparations, such as establishing Class IV and Class V supply points and start or completion
  times of obstacle belts and groups. Commanders use artillery, air, or ground systems to reseed minefields
  that an enemy has breached. (See Chapter 17 for further information on combined arms countermobility
  operations.)
  Mobility
  8-75. During the defense, mobility tasks include maintaining routes, coordinating gaps in existing obstacles,
  and supporting counterattacks. Engineers also open helicopter landing zones and tactical landing strips for
  fixed-wing aircraft. Maintaining and improving routes and creating bypass or alternate routes at critical points
  are major engineering tasks because movement routes are subject to fires from enemy artillery and air support
  systems. These enemy fires may necessitate deploying engineer equipment forward, such as assault bridging
  and bulldozers. Units can also evacuate or restrict movement of dislocated civilians to routes not required by
  friendly forces to avoid detracting from the mobility of the defending force. A unit’s coordination with
  available civil affairs forces, intergovernmental and nongovernmental entities, and potentially local nation
  government officials can also assist units in preserving their combat power.
  8-76. Typically, a unit’s priority of mobility support is to routes used by counterattacking forces, then to
  routes used by main body forces displacing to subsequent positions. Mobility support mainly involves
  reducing obstacles and improving or constructing combat roads and trails to allow tactical support vehicles
  to accompany moving combat vehicles. Units coordinate carefully to ensure units leave lanes or gaps in
  obstacles for repositioning main body units and committing the counterattack force. CBRN reconnaissance
  systems also contribute to the force’s mobility in a contaminated environment.
  Obscuration
  8-77. Units use obscuration to disrupt the enemy’s assault or movement formations and to deny the enemy’s
  use of target acquisition optics, visual navigation aids, air avenues of approach, landing zones, and drop
  zones. Obscuration creates gaps in enemy formations, separates or isolates attacking units, and disrupts
  enemy planned movements. Bispectral obscuration can blind attackers who lack thermal viewers or other
  enhanced optical systems. It prevents overwatching enemy elements from observing and engaging the
  defender, while defending forces with advanced optical systems can acquire and engage the enemy within
  the smoke. The commander can use smoke to facilitate friendly target acquisition by highlighting enemy
  systems against a light background while degrading the enemy’s optics. Smoke used to mask obstacles
  located in low-level flight corridors and on landing and drop zones can prevent an enemy from using these
  drop zones or greatly increase the enemy’s risk.
  8-78. Units use smoke-generation capabilities to mark targets and screen or obscure friendly positions. They
  must carefully employ obscurants with regard to enemy systems and friendly capabilities. Improper use can
  create an advantage for the enemy. The effectiveness of smoke depends on weather conditions and the
  quantity of smoke employed. Units coordinate the use of smoke generators, artillery or mortar smoke, and
  smoke pot employment. These smoke-producing systems are most effective when used together to achieve
  synergistic effects. Using smoke can also enhance military deception operations and cover friendly
  movements. (See ATP 3-11.50 for more information on planning, preparing, and executing battlefield
  obscuration.)