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Chapter 4 4-10 ADP 3-90 31 July 2019 positions in selected engagement areas. In a mobile defense, commanders use a striking force to generate overwhelming combat power at the decisive point. Commanders can also mass effects by committing the reserve. Commanders use economy of force measures in areas that do not involve the decisive operation. Armored and Stryker Forces 4-57. When most of a defending force consists of units equipped with armored combat vehicles, commanders can conduct a defense designed to take advantage of the tactical mobility and protection offered by those systems. Combat vehicles provide defending forces with the capability to maneuver to delay the advance of a strong enemy force and then immediately change from a dynamic to a static defense or counterattack. Defending forces equipped with armored combat vehicles are well suited for use as security and MBA forces. They are more suited for operations within a chemical, biological, radiological, and nuclear (CBRN) contaminated environment than dismounted infantry forces because of their built-in CBRN overpressure protection. Dismounted Infantry Forces 4-58. Dismounted infantry forces facing an armored enemy force are primarily used in static roles within the MBA or in security roles within their echelon support and consolidation areas. When facing armored enemy forces, dismounted infantry forces are most effective when fighting from prepared defenses or in close terrain, such as swamps, woods, hilly and mountainous areas, and urban areas. From those positions they can take advantage of their foot mobility and short-range infantry and anti-armor weapons. 4-59. Commanders use airborne and air assault units in the same manner as they use other infantry forces once those units deploy into their landing zones. However, there may be challenges extracting them, particularly if those units come into direct contact with an enemy force. Because of its mobility and potential reaction speed, an airborne or air assault force is often well suited for a reserve role during the defense. Its tasks might include—  Rapid reinforcement of a threatened position.  Occupation of a blocking position, possibly in conjunction with existing defensive positions.  Reinforcement of encircled friendly forces.  Flank protection. Aviation Forces 4-60. Aviation forces with their mix of manned and unmanned systems are particularly valuable in the defense because of their speed, mobility, and versatility. Their tasks can include—  Conducting intelligence, surveillance, and reconnaissance, as part of information collection.  Performing security operations.  Conducting shaping operations or supporting efforts to establish the necessary conditions for decisive operations by other forces or the main effort through attriting, disrupting, and delaying the enemy.  Conducting counterattacks and spoiling attacks.  Controlling ground for limited periods where a commander does not wish to irrevocably commit ground forces; for example, forward of an executed obstacle.  Countering enemy penetrations.  Closing gaps in a defensive plan before the arrival of ground maneuver forces.  Facilitating the disengagement of ground forces.  Countering enemy activities in the echelon support areas, in particular enemy airborne or air assault forces.  Resupplying a defending force or facilitating casualty evacuation.  Assisting in the countermobility effort by employing minefields.