French Forces of the Interior

The French Forces of the Interior (French: Forces Françaises de l'Intérieur) refers to French resistance fighters in the later stages of World War II. Charles de Gaulle used it as a formal name for the resistance fighters. The change in designation of these groups to FFI occurred as France's status changed from that of an occupied nation to one of a nation being liberated by the Allied armies. As regions of France were liberated, the FFI were more formally organized into light infantry units and served as a valuable manpower addition to regular Free French forces. In this role, the FFI units manned less active areas of the front lines, allowing regular French army units to practice economy of force measures and mass their troops in decisive areas of the front. Finally, from October 1944 and with the greater part of France liberated, the FFI units were amalgamated into the French regular forces continuing the fight on the Western Front, thus ending the era of the French irregulars in World War II.

After the invasion of Normandy in June 1944, at the request of the French Committee of National Liberation, SHAEF placed about 200,000 resistance fighters under command of General Marie Pierre Kœnig, who attempted to unify resistance efforts against the Germans. General Eisenhower confirmed Koenig's command of the FFI on 23 June 1944.

The FFI were mostly composed of resistance fighters who used their own weapons, although many FFI units included former French soldiers. They used civilian clothing and wore an armband with the letters "F.F.I."

...
Wikipedia