General Fritz von Lossberg (1868--1942) directed virtually all the major German defensive battles on the Western Front during the First World War. Hailed as "the Lion of the Defensive," he was an extremely influential military tactician and, unlike many other operations officers of his era, was quick to grasp the changes wrought by technology. Now available for the first time in English, Lossberg's memoir explains how he developed, tested, and implemented his central principles -- flexibility, decentralized control, and counterattack -- which were based on a need to adapt to shifting conditions on the battlefield. Lossberg first put his theory of elastic defense combined with defense-in-depth into practice during the Battle of Arras (April--May 1917), where it succeeded. At the Battle of Passchendaele (June--November 1917), his achievements on the field proved the feasibility of his strategy of employing a thinly manned front line that minimized the number of soldiers exposed to artillery fire. Lossberg's tactical modernizations have become essential components of army doctrine, and Lossberg's War: The World War I Memoirs of A German Chief of Staff will take readers inside the mind of one of the most significant military innovators of the twentieth century.