Sir William Robertson served as the professional head of the British army and as the constitutional military adviser to both Asquith and Lloyd George from December 1915 to February 1918. This account, based on many new sources, critically examines his leadership of the general staff as the burden of fighting the main body of the German army shifted to the British. This study sheds light on the origins and conduct of the Somme and Passchendaele offensives, and the efforts to coordinate the Allied war effort, especially the controversial effort to subordinate Haig to General Nivelle and the creation of the Supreme War Council with its inter-allied staff. The civil-military conflict over the conduct of the war, especially the growing divide between Robertson and Lloyd George, receives special attention. The previously unexplored tension between Robertson and Haig who formed the most important military partnership in British history is also examined. This account represents the untold story of the higher direction of the war in Britain.