When U.S. President Franklin D. Roosevelt and British Prime Minister Winston Churchill met at the Arcadia Conference (December 1941January 1942), they began a period of wartime cooperation that, for all the very serious differences that divided the two countries, remains without parallel in military history. Anglo-American cooperation was formally embodied in the Combined Chiefs of Staff, which was not so much a body as a system of consultation, reinforced by frequent conferences, between the British Chiefs of Staff Committee and the U.S. Joint Chiefs of Staff. Between conferences, the British Joint Staff Mission, based in Washington, D.C., maintained contact with the U.S. Joint Chiefs of Staff on behalf of their counterparts in the United Kingdom.
For the invasion of northwest Europe, the Combined Chiefs created the temporary position of Supreme Commander Allied Expeditionary Force and assigned it to General Dwight D. Eisenhower, an American with a proven ability to work amicably with the often considerable personalities who directed the Allied armies in Europe. Eisenhower's Supreme Headquarters Allied Expeditionary Force (SHAEF) had authority over all the branches (air, sea, and land) of the armed forces of all countries whose contribution was necessary to the success of Operation Overlord (the planned Normandy invasion). These were grouped for the invasion under the Allied Naval Expeditionary Force, the Allied Expeditionary Air Force, and the Twenty-first Army Group (the expeditionary ground force)all commanded by Britons. For the duration of Overlord, the U.S. Strategic Air Forces in Europe and the Royal Air Force Bomber Command were placed directly under the supreme commander's authority, ensuring the contribution of those very important commands to the overall invasion plan. The European Theater of Operations, U.S. Army, was to direct the gigantic effort of supplying an entire invasion army as it crossed the English Channel and advanced into the Continent. French General Charles de Gaulle, president of the French Committee of National Liberation but by no means the universally acknowledged head of the French government-in-exile, maintained a liaison with SHAEF through the commander of the Free French Forces in Britain.
Below the level of expeditionary force or army group, the various air forces, naval task forces, and armies were divided into British or American commands (the First Canadian Army achieving coequal status during the Normandy campaign). Even at the operations level, however, the cooperation among the fighting units reflected the binational structure of SHAEF and the Combined Chiefs of Staff. In this manner the Anglo-American allies managed to avoid the division of responsibility that was built into the German chain of command and that proved fatal to the Germans' war effort from D-Day on.