This book sets out the true role of British Corps (and their commanders) and crucially their control of artillery, which led to their becoming the principal operational level of command on the Western Front in the BEF. At the start of the Great War, the corps functioned as a postbox, there to help GHQ manage its divisions. From early 1916 onwards, corps took control of both heavy artillery and divisional artillery and, vitally, the counter-battery role. In 1917, building on the lessons of the Somme via the SS series of pamphlets, and especially SS135, corps increasingly became the level of command which organized attacks and orchestrated the artillery effort and divisions' infantry plans. In 1918, learning lessons in open warfare, the BEF was sufficiently flexible for corps to coordinate only when a set piece was required, and devolve command forward to divisions if circumstances permitted it. This book also examines the decision making process in the BEF and concludes that at the Army and corps level it was neither "umpiring" nor unduly authoritarian. This is the first book ever to carry out an examination of how Great War British generals actually carried out their role?how corps commanders commanded their corps on a day-to-day basis.