Military reserve 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 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 . 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 . Background . In concept , the idea dates back in antiquity to ca . the sixth century BC , at least as far as the early Roman legion ary 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 . Reserves at various levels . 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 . Employment . 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 . Effect of reserves in history . 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 . Example of reserves . There have been many examples of reserves formations throughout history . British Reserve Army in World War I . 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 . Strike Corps . Some armour heavy formations of India and Pakistan are designated as " strike corps " to take advantage of breakthroughs .