In the days before the war, there was in the British Service no recognized or authorized organization known as an Army Corps. When the Expeditionary Force was launched into the conflict in 1914, the Army Corps organization was hastily improvised, and consisted at first merely of an Army Corps Staff, with a small allotment of special Corps Troops and services, and of a fluctuating number of Divisions. It was the _Division_[1] and not the _Corps_, which was then the strategical unit of the Army. Even when the necessity for the formation of Army Corps was recognized, it was still a fundamental conception that it was the Division, and not the Army Corps, which constituted the fighting unit. To each Army Corps were allotted at first only two, but later as many as four Divisions, according to the needs and circumstances of the moment. But the component Divisions never, for long, remained the same. The actual composition of every Army Corps was subject to constant changes and interchanges, and it was rare for any given Division to remain for more than a few weeks in any one Army Corps. The disadvantages of such an arrangement are sufficiently obvious to require no great elaboration; at the same time, it has to be recognized that, during the first three years of the war, at any rate, the Army was undergoing a process of rapid expansion, and that, on grounds of expediency, it was neither possible nor desirable to adopt a policy of a fixed and immutable composition for so large a formation as an Army Corps. Moreover, the special conditions of trench warfare made it imperative to create, under the respective Armies, and in the respective zones of those Armies, a subordinate administrative and tactical authority with a more or less fixed geographical jurisdiction. Thus, the frontage held by each of the five British Armies became subdivided into a series of Corps frontages, and each Corps Commander had allotted to him a definite frontage, a definite depth and a definite area, for his administrative and executive direction.