[1] The 15th Army Group was activated in 1943 in Algiers, North Africa, to plan the invasion of Sicily, codenamed Operation Husky.

Despite and due to the rapid advance of the Allied forces in Italy in June–July 1944, after the liberation of Rome, the high allied command in Western Europe decided to take away from the Italian front the French Expeditionary Corps and the U.S. VI Corps, reassigned for landing in the South of France in support of the advance in the north of that country, and to liberate southern France, including the huge port complexes of Marseilles and Toulon, and bring into action the seven divisions of the French 1st Army (1st and 5th armored, 1st, 2nd, 3rd, 4th, and 9th infantry) that had been reequipped in French North Africa by the United States.

Additional replacements and service elements were provided by conversion of the US Army's 2nd Cavalry Division, which had arrived in the theater in 1944.

The new gaps on the Italian front was filled by four Italian "combat groups," each equivalent to a "light" (two brigade) division under the British table of organization and equipment, additional US troops (detached from the forces in France or converted from army-level cavalry and anti-aircraft units) and one additional brigade made up largely of infantry recruited in the British Mandate of Palestine.

[4] In December 1944, Alexander was made Supreme Commander of the Allied Forces Headquarters responsible for all military operations in the Mediterranean theatre and Lieutenant General Mark Clark became the new commander and the army group was renamed 15th Army Group once again.