Reserve formation

A military reserve, reserve formation, or simply reserve, is a group of military personnel or units which are initially not committed to a battle by their commander so that they are available to address unforeseen situations or exploit sudden opportunities. Such forces may be held back to defend against attack from other enemy forces, to be committed to the existing battle if the enemy exposes a vulnerability, or to serve as relief for troops already fighting. Some of the different categories of military reserves are: tactical reserve, operational reserve, strategic reserve.

A military reserve is different from a military reserve force, which is a military organization composed of military personnel who maintain their military skills and readiness in a long-term part-time commitment to support their country if needed. Military reserve refers to specific trained pre-organized forces operating as an on call basis from the main military force.

The concept of reserve forces dates at least to early Roman legionary formations. In the early Roman legion, the soldiers were divided by wealth and age, with the poorer and younger Hastati forming the first line, followed by the Principes, and the wealthier and older Triarii. The first two lines were expected to do the majority of the fighting, with the third line held in reserve, both as a last resort and to deter routing.

In the modern battlefield, reserves exist at all levels, from a platoon held back from a company level engagement, to whole army corps consisting of armoured and mechanised divisions which are held in reserve with the purpose of exploiting a breakthrough or containing an enemy advance. Typically what is a reserve for one headquarters is not the reserve for a higher headquarters (though depending on the setup they may be). So if one of a battalion's companies is held is reserve during a battle, the company is considered to be a reserve for the battalion but not for the brigade or the division, since it is committed to action in its parent battalion sector. Similarly the British Reserve Army of World War I and the American Fifteenth Army of World War II were only reserves in their theater, as far as the national Headquarters was concerned, they were committed since they were not available to be sent for action in any other theater.

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