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A military reserve, tactical reserve, or strategic reserve (and normally called only as the/his/their "Reserve/(s)" or sometimes as a "reserve formation") 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 suddenly developing 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.
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.
In concept, the idea dates back in antiquity to ca. the sixth century BC, at least as far as the early Roman Legionary formations when their lines were organized with newer less qualified soldiers in the front ranks backed by a wily group of experienced veterans. Earning the right and having the experience to stand in the reserve or third rank was considered an achievement and promotion in the Roman culture. On battlefields it was discovered that it was often better to have a group of soldiers held away from the line, to deal with weak points or opportunities, since it is very difficult for a unit or formation which is in action to extricate itself in good order and then refocus to the new assignment. This also causes the situation in the sector that unit was fighting in originally to become worse. By using uncommitted reserves the commander could continue his operations in all sectors and quickly move to reinforce weak points or exploit opportunities.
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.
The decision when and where and how to employ the reserves is one of the most important decisions a commander makes. Usually only a part of the reserves are utilised at any given time, since these are often sufficient to accomplish the task at hand. The "all reserves forward" order is given only in times of extreme crisis where it is clear the enemy will not be stopped otherwise. An example of this was on one of the days in the Battle of Britain. In the event of reserves being sent forward to exploit a breakthrough, usually some are kept back to deal with any counterattack. Reserves can also be employed to relieve troops, thus allowing for a rotation ensuring all troops get adequate rest.
Reserves may also exist in a global sense, as in instead of being tasked as the reserve in some area, units and formations may be held back as forces which could be sent to potential new hotspots, or held back for a decisive battle, as the battleship Yamato was.
Reserve troops (or lack thereof) have played a big role in battles and campaigns throughout history, especially in the twentieth century. The presence of reserves in France and southern Europe allowed Nazi Germany to quickly rush forces to Tunisia to deal with the aftermath of Operation Torch, ensuring that there would be no collapse, as well as in Italy, to speedily take over most of the country after the Allied landing and the Italian surrender. Conversely, the lack of reserves to deal with the Allied landings in Sicily compelled the Germans to move troops from the Eastern Front (which included units and formations then heavily engaged) causing the German offensive at Kursk to be adversely affected, and the fact that in the Battle of the Bulge many of Germany's reserves had been expended, contributed heavily to the general German collapse in 1945.
In the aftermath of World War I, the extensive colonial commitments of the United Kingdom ensured that few battalions were available for the Anglo-Irish War, which decreased the British ability to deal with the crisis.
At the Battle of Gaugamela, Alexander the Great's cavalry which had been held back, first dealt with outflanking movements of the Persians and later when a gap opened up broke through to win the battle.
There have been many examples of reserves formations throughout history.
The Reserve Army was a field army of the British Expeditionary Force during the First World War. Under the command of Lieutenant General Sir Hubert Gough, the Reserve Army was formed on 23 May 1916 prior to the Battle of the Somme and was renamed the Fifth Army in October of that year.
The intended purpose of the army was to carry out the breakthrough phase of the Somme offensive once General Sir Henry Rawlinson's Fourth Army had captured the German front-line trenches. For this role Gough was provided with the three British cavalry divisions and in June he was allocated an infantry corps of three divisions to support the advance.
The United States Army's III Corps is a dedicated counter-offensive corps.
Some armour heavy formations of India and Pakistan are designated as "strike corps" to take advantage of breakthroughs.