Eisenhower transferred from command of the Mediterranean Theater of Operations to command SHAEF, which was formed in Teddington, London from December 1943, in what is now Bushy Park; a street named Shaef Way remains to this day. Its staff took the outline plan for Operation Overlord created by Lieutenant General Sir Frederick E. Morgan, COSSAC (Chief of Staff to the Supreme Commander Allied Forces), and Major General Ray Barker. Morgan, who had been appointed chief of staff to the supreme Allied Commander (designate) in mid-March 1943 began planning for the invasion of Europe before Eisenhower's appointment. and moulded it into the final version which was executed on 6 June 1944. That process was shaped by Eisenhower and the land forces commander for the initial part of the invasion, General Sir Bernard Law Montgomery.
SHAEF remained in the United Kingdom until sufficient forces were ashore to merit its transfer to France. At that point, Montgomery ceased to command all land forces, but continued as Commander in Chief of the British 21st Army Group (21 AG) on the eastern wing of the Normandy bridgehead and the American 12th Army Group (12 AG) commanded by Lieutenant General Omar Bradley was created as the western wing of the bridgehead. As the breakout from Normandy took place, the Allies launched the invasion of southern France on 15 August 1944 with the American 6th Army Group (6 AG) under the command of Lieutenant General Jacob L. Devers. During the invasion of southern France, the 6 AG was under the command of the Allied Forces Headquarters (AFHQ) of the Mediterranean Theatre of Operations, but after one month command passed to SHAEF. By this time the three Army Groups had taken up the positions on the Western Front in which they would remain until the end of the war—the British 21 AG to the North, the American 12 AG in the middle and the 6 AG to the South. By December 1944, SHAEF had established itself in the Trianon Palace Hotel in Versailles, France. On 26 April 1945 SHAEF moved to Frankfurt.
SHAEF commanded the largest number of formations ever committed to one operation on the Western Front, with American, French army of liberation, British and Canadian Army forces. It had three Army Groups under its command, which controlled a total of eight field armies;
SHAEF also controlled substantial naval forces during Operation Neptune, the assault phase of Overlord, and two tactical air forces: the US Ninth Air Force and the RAF Second Tactical Air Force. Allied strategic bomber forces in the UK also came under its command during Operation Neptune.