On July 24, 1943, following the Allied invasion of Sicily, the Italian fascist Prime Minister Benito Mussolini was arrested and a provisional Italian government, under Marshal Pietro Badoglio was established. Opposed to the alliance with Nazi Germany, Badoglio signed an armistice with the Allies.
With King Victor Emmanuel III and Marshal Badoglio in command, the Royal Italian Army entered the war on the side of the Allies. The Royal Army soon faced a crisis of leadership, however, and a swift German occupation of Italy and Italian positions in the Balkans resulted in >70,000 casualties within a few months.
After his release by the Germans in September 1943, Mussolini established the Republica Sociale Italiana, a puppet state of Germany in northern Italy that existed until the end of the war.
Adolf Hitler, concerned that the Allies would establish air bases in Italy that could threaten southern Germany and important oil supplies in Romania, ordered the deployment of 16 new divisions to the Italian mainland. The subsequent advance of the Allied Italian campaign was costly and prolonged. The last German forces in Italy surrendered in May 1945.