Britain's conduct in the First World War was largely dependent on coalitions and international co-operation, particularly with France. The allies' relationship was fractured but Britain's primary goal, throughout the war, was its preservation. The Salonika Campaign, based on the Macedonia front, posed the biggest threat to their coalition. Despite it having the smallest impact on the outcome of the War, this Campaign weakened the collaboration of the two armies on the Western Front. Drawing on previously unseen archival material, this work examines the Salonika Campaign as central to Britain and France's wartime alliance.