France had declared war on Germany on 3 September 1939 at the same time as the United Kingdom, however, the country was overrun during the campaign of April and May 1940. Under the terms of the Armistice, a pro-German government was formed, based at the town of Vichy. The French possessions of Morocco, Algeria, Tunisia, Syria, Madagascar all aligned themselves with the new Vichy government.

The French forces put up some token resistance to the Allied invasion of North Africa in November 1943. An armistice was signed on 11 November 1942, and the French forces in North Africa joined the Allied cause. A new French Army was formed, using American arms and equipment.

In early 1943, three divisions were formed in Algeria and two were formed in Morocco. The Free French Division which had seen active service in Syria, Egypt, Libya and Tunisia also joined the new French Army. The higher level formation was originally designated as the 1ere Armee Francaise, but this was perceived to be problematic as it would serve under the Headquarters 5 U.S. Army. It was decided to designate the formation as the Corps Expeditionnaire Francais (French Expeditionary Corps). The corps had under command four French divisions, which transferred to Italy one by one. The corps headquarters also moved to Italy where it became operational on 18 May 1943.

The French Expeditionary Corps played a key role in the assault on the Gustav Line and Battle for Monte Cassino. The entire corps was then withdrawn from Italy for preparation for Operation Dragoon, the landings in Southern France. The corps expanded to form the French 1 Army and landed in Southern France on 15 August 1944.

Four divisional formations served in Italy, the:

• 1 Division of Foot Infantry (formerly the Free French Division),
• 2 Moroccan Infantry Division,
• 3 Algerian Infantry Division,
• 4 Moroccan Mountain Division.

The majority of officers and senior non-commissioned officers in all four divisions were French, with men from the French colonies forming the bulk of the soldiers in each formation.