The term reserves is used in two senses in the military.

Combat Reserves. In the first sense, reserves are ground combat forces held back from the front as a hedge against the unexpected. They can be rushed to a point threatened by a sudden enemy attack, or they can be used to exploit an opportunity created by one's own offensive actions. A common rule of thumb was that a third of one's combat strength at each echelon should be held in reserve whenever possible ("two up, one back"). Thus, an infantry platoon ideally deployed two of its squads up front and held one a few tens of yards (meters) back, and so on up to the level of a corps, which might deploy two divisions on the line and hold a third some miles (kilometers) back from the front. In practice, combat units were often hard-pressed to maintain much of a reserve. A small but mobile reserve, such as a tank or cavalry unit, was sometimes called a "fire brigade" when employed in a desperate defensive situation, since its job was to rush from point to point to stabilize the line.

An important part of the operational art in warfare was knowing when and where to commit one's reserves. It was also important to know where to position one's reserves. Reserves deployed well away from the front were in a better position to rest, resupply, and absorb replacements, and they had maximum room to maneuver when called on. However, reserves deployed closer to the front could usually march into action more quickly when called up. Here the rule of thumb was the equilateral triangle: In the ideal "two up, one back" case, if the forward units were covering a front of a mile, the reserve should be between half a mile and a mile back.

One of the most serious failings of Chinese strategy and tactics was a tendency to try to cover all points evenly, leaving inadequate reserves. Since Chinese forces were almost completely lacking in transport, their ability to move reserves to threatened points was very limited in any case.

Maintaining a reserve is also important in air combat. In the Japanese Navy of 1941, the practice was to keep three fighters from each carrier in the air as a combat air patrol; three more fighters spotted on deck for immediate launch if needed; and a final trio of fighters ready to be spotted for launch within a few minutes. American practice was heavily influenced by radar, which allowed an approaching enemy raid to be detected and evaluated in time to vector an appropriate number of fighters, and launch additional fighters, if required. The fighter control officer had to take into account the fuel status of his aircraft and the likelihood of new enemy raids materializing when allocating fighters against incoming raids.

Reserves are not a useful concept in surface naval warfare. The danger of defeat in detail mandates that the admiral commanding a battle fleet hold nothing back.  It was likewise the practice of ground forces not to place any of their artillery in reserve. When a unit with organic artillery went into reserve, its artillery would be placed under the command of the parent unit artillery commander to continue supporting the parent unit.

Reservists. It is expensive and wasteful to keep large numbers of troops on active duty in peacetime. On the other hand, basic training of new troops takes time and resources that may be in short supply when war suddenly breaks out. As a result, all the armies involved in the Pacific War had trained significant numbers of reserve troops who then returned to civilian life until called for. In the Army of the United States, these reservists fell under either the National Guard or the Army Reserve. Both the Army and the Navy also trained reserve officers who were subject to mobilization in time of war.

Reservists were not well regarded by the Japanese Army, which believed that men with wives and children lacked the spiritual power so emphasized in Japanese military doctrine. However, reservists were used in increasing numbers in China from late 1937 on to free the regular troops for service in Manchuria or training for a war against Russia. By 1938 reservists constituted 45% of forces on active duty. These troops were charged with four times as many criminal offenses as regular troops in the first two years of the China Incident, suggesting that there was some basis for the Army's attitude towards them. By 1940 reservists had been reduced to 28% of the active duty force.

Japanese reservists were divided into the first reserve of men aged 24-28, the second reserve of men aged 29-34, and the conscript reserve of poorly trained personnel used as service troops.


Drea (2009)

FM 6-20 (1944-2-5; accessed 2012-7-9)

Hsiung and Levine (1992)

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