This book is the first critical study of the Anglo-French political and military relationship in the First World War, focusing on joint military operations in the main theatre, the western front. It examines the strategic principles which lay behind military policy in this theatre in the context of a close alliance in which divergent strategies, institutional weaknesses and clashing personalities had a detrimental effect on the coordination of military policy. Within this context it looks at the problems of strategic coordination in the 'war of movement' in 1914, the planning of the costly Somme and Passchendale offensives, and the factors which finally led to the effective coordination of allied strategy and military effort by General Foch in 1918. By so doing it throws new light on the reasons why the allies failed to win a decisive military victory on the western front before 1918.