SHAEF was created in December 1943 when Eisenhower was recalled from the Mediterranean Theatre of Operations to the United Kingdom. Its staff took the outline plan for Operation Overlord created Lieutenant General Sir Frederick Morgan, COSSAC (Chief of Staff to the Supreme Commander Allied Forces) 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 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, merely becoming one of three Army Group commanders under Eisenhower. SHAEF commanded the largest number of American formations ever committed to one operation, along with substantial French, British and Canadian forces. It had three Army Groups under its command, which controlled a total of eight field armies (five American, one French, one British, one Canadian). SHAEF also controlled substantial naval forces during Operation Neptune, the assault phase of Overlord, and two tactical air forces. The strategic bomber forces in the UK came under its command during Operation Neptune.