It was the highest rank in the theater, but hasn't been used since WWII. It shares a common lineage with Supreme Allied Commander Europe and Atlantic, but they are different titles.
The SupremeAlliedCommand in Europe would never have worked without Eisenhower, for he virtually invented the concept of Allied unity of command and persuaded the British to accept it in lieu of the committee system to which they were accustomed.
Alliedcommanders learned something of the difficulties of fighting alongside each other, and the entire Alliedforce gained invaluable experience in planning and conducting amphibious landings.
As Alliedcommander, he was not only a military leader but also the representative of the Combined Chiefs of Staff and their respective governments when such political issues as the handling of the Vichy regime had to be resolved.
In December 1943 he was appointed SupremeCommander, AlliedExpeditionaryForces, charged with planning and carrying out the Allied invasion of France, Operation Overlord, in June 1944.
He negotiated with Soviet commanders such as Marshall Zhukov, and sometimes directly with Stalin, such was the confidence that President Roosevelt had in him.
Eisenhower was named Chief of Staff of the U.S. Army in November 1945 and in December 1950 he was named SupremeCommander of the North Atlantic Treaty Organization, and given operational command of NATO forces in Europe.