This paper aims to show that the Allies of the Second World War formed a major, yet largely overlooked site of mid-twentieth-century internationalism. Historians have tended to tell national stories about this coalition, built around its leaders and their bilateral relationships and conflicts, such as the 'special' Anglo-American relationship between Churchill, Roosevelt and their generals. The present study, by contrast, foregrounds the military and civilian planners working underneath them, as well as the inter-Allied institutions that facilitated their cooperation. This is a half-forgotten chapter of the war's history: a series of technical, so-called 'combined' organs designed to plan Allied grand strategy and operations, pool their productive resources, and unify their theatre-level military commands, set up by Great Britain and the United States after the latter's formal entry into the war in December 1941. The planners serving on these boards and committees, the paper furthermore shows, described their work in explicitly internationalist terms. 'Combination', as these insiders called it, meant putting the objective needs of Allied strategy ahead of narrow national interests. Its history is more than merely Anglo-American: involving Canadian, French and other European actors, it encompassed the wider transAtlantic and foreshadowed the later Atlantic alliance. Indeed, since several key shapers of postwar European integration, notably Jean Monnet, were closely involved in the combined experiment, this paper shines new light on a warlike root of European cooperation. Thus, it opens a dialogue between the history of war, internationalism and Europeanisation.