However, Italy's conquests were always heavily contested, both by various insurgencies (most prominently the Greek resistance and Yugoslav partisans) and Allied military forces, which waged the Battle of the Mediterranean throughout and beyond Italy's participation. The country's imperial overstretch (opening multiple fronts in Africa, the Balkans, Eastern Europe, and the Mediterranean) ultimately resulted in its defeat in the war, as the Italian empire collapsed after disastrous defeats in the Eastern European and North African campaigns. In July 1943, following the Allied invasion of Sicily, Mussolini was arrested by order of King Victor Emmanuel III, provoking a civil war. Italy's military outside of the Italian peninsula collapsed, its occupied and annexed territories falling under German control. Italy capitulated to the Allies on 3 September 1943. On 13 October 1943, the Kingdom of Italy officially joined the Allied Powers and declared war on it former Axis partner Germany.