On September 3, 1943, representatives of the Italian government signed an armistice with the Allies at Cassibile in Sicily, ending Italy's war against Britain and the United States. The signing was kept secret and announced publicly on September 8, at the same time as the Allied landings near Salerno.
King Victor Emmanuel III had had Mussolini arrested on July 25, and Marshal Pietro Badoglio's government then opened secret talks while German divisions were moving into northern Italy. The terms were unconditional. Italy had to withdraw its forces from the fighting, hand over its fleet, and let the Allies use its territory and airfields.
German commanders had already drafted plans to occupy the peninsula and disarm the Italian army, and those five days of secrecy gave them the time they needed to move in more divisions and take up positions near every major garrison. When the armistice was read out on September 8, the Italian army largely dissolved. Rome fell within days. The King and Badoglio fled to Brindisi.
Italy declared war on Germany on October 13. The peninsula became a battlefield for twenty months. German paratroopers freed Mussolini in September and installed him as head of the Italian Social Republic in the north. It ended in April 1945.