6-145. Commanders use reserves to counterattack, to exploit enemy weaknesses such as exposed flanks or units vulnerable to defeat in detail, or deny the enemy control of critical terrain. The reserve can also reinforce forward defensive positions, contain enemy penetrations, or react to threats in the support or consolidation areas. Commanders decide on the size, composition, and mission of the reserve as early as possible after determining what risk is acceptable. Commanders down to the BCT level normally retain one third of their maneuver strength in reserve; the higher the degree of uncertainty regarding the enemy, the larger the reserve should be.