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Chapter 2 2-16 ADP 3-90 31 July 2019 normally the echelon’s main effort once it is committed. Commanders constitute a reserve and base the size of the reserve on the level of uncertainty in the current tactical situation. Commanders consider survivability and the most likely mission when positioning their reserve. While commanders can assign their reserve a wide variety of tasks to perform on commitment, a reserve remains prepared to accomplish other missions. The primary tasks for a reserve are to—  Retain the initiative.  Take advantage of unexpected success.  Counter tactical reverses that threaten the integrity of the friendly force’s operations. A successful commander retains a reserve, reconstituting one whenever possible on the commitment of the original reserve. RULES OF ENGAGEMENT 2-74. Rules of engagement are directives issued by competent military authority that delineate the circumstances and limitations under which United States forces will initiate and/or continue combat engagement with other forces encountered (JP 3-84). Operational requirements, policy, and law define the rules of engagement a commander must follow. Rules of engagement influence how a commander conducts operations by imposing political, practical, operational, and legal limitations. They may extend to criteria for initiating engagements with certain weapon systems, such as employing unobserved indirect fires within the echelon support and consolidation areas, or reacting to an attack. Unit commanders always retain the inherent right and obligation to exercise unit self-defense in response to a hostile act or demonstrated hostile intent. Unless otherwise directed by a unit commander, military members may exercise individual self-defense in response to a hostile act or demonstrated hostile intent. The Joint Chiefs of Staff have established standing rules of engagement. Operational-level commanders modify those standing rules of engagement as necessary. (See FM 1-04 for additional information on rules of engagement.) SUPPORTING DISTANCE 2-75. Supporting distance is the distance between two units that can be traveled in time for one to come to the aid of the other and prevent its defeat by an enemy or ensure it regains control of a civil situation (ADP 3-0). For lower echelon units, it is the distance between two units that can be covered effectively by their fires. Supporting distance is a factor of combat power, dispositions, communications, and the tactical mobility of friendly and enemy forces. SUPPORTING RANGE 2-76. Supporting range is the distance one unit may be geographically separated from a second unit yet remain within the maximum range of the second unit’s weapons systems (ADP 3-0). Major factors that affect supporting range are terrain, the range of the supporting unit’s weapon systems, and the locations of weapons systems in relation to a supported unit’s position. TACTICAL MOBILITY 2-77. Tactical mobility is the ability of friendly forces to move and maneuver freely on the battlefield relative to the enemy. Tactical mobility is a function of the relationship of cross-country mobility, firepower, and protection. The terrain, soil conditions, and weather affect cross-country mobility. Armored ground maneuver units, such as combined arms battalions, have good tactical mobility—except in restrictive terrain—combined with maximum firepower and protection. Infantry ground maneuver units, such as airborne infantry battalions, have a tactical mobility advantage against enemy armored forces in restrictive terrain, but they have limited firepower and protection. Stryker equipped forces also have good tactical mobility, but they possess less firepower and protection than armored forces. Army aviation maneuver units have excellent mobility in all but the most restrictive types of terrain, but they have limited protection. Extreme weather conditions also restrict the mobility of Army aviation units.