The Italian Co-Belligerent Air Force (Aviazione Cobelligerante Italiana, or ACI), or Air Force of the South (Aeronautica del Sud), was the air force of the Royalist "Badoglio government" in Southern Italy during the last years of World War II. The ACI was formed in Southern Italy in October 1943 after the Italian Armistice in September. As by this point the Italian Kingdom had defected from the Axis and had declared war on Germany, the ACI pilots flew for the Allies.
A small part of the Italian Royal Air Force (Regia Aeronautica) remained under German control. This was known as the National Republican Air Force (Aeronautica Nazionale Repubblicana, or ANR), ostensibly part of the forces of Benito Mussolini's Fascist state in northern Italy, the Italian Social Republic (Repubblica Sociale Italiana). The ANR pilots flew with the Axis.
By the end of 1943, 281 Italian warplanes had landed at Allied airfields, but most were no longer useful for combat. The crews of these aircraft were re-equipped with Allied aircraft and engaged in transport, escort, reconnaissance, sea rescue, and limited tactical ground support operations flying 11,000 missions from 1943 to 1945.
The ACI never operated over Italian territory, its objectives being always in the Balkans (Yugoslavia or Albania). This was to avoid any possible encounter between Italian-crewed aircraft fighting on opposite sides. During the entire history of ACI, no encounter, let alone combat, was ever reported between ACI and ANR aircraft.
The ACI formed the basis of the post-war Air Force of the Italian Republic (Aeronautica Militare Italiana).
thumb|A Reggiane Re.2002 of the 293ª Squadriglia of the Italian Co-Belligerent Air Force