The Italian Civil War (8 September 1943 – 2 May 1945) was a sub-conflict of World War II in which the 350,000-strong Italian Resistance and the 190,000-strong Italian Co-belligerent Army fought together to defeat the 600,000-strong army of the fascist Italian Social Republic and its German allies. The civil war pitted the communist, socialist, anarchist, and liberal partisans and the royalist Co-belligerent Army against the German Wehrmacht occupiers and the armed supporters of Benito Mussolini, and several massacres occurred both during the war and in its immediate aftermath. The war ended with the execution of Mussolini and several other fascist leaders in April 1945 and the German surrender in Italy on 2 May 1945.
The conflict occurred as a result of the Armistice of Cassibile on 8 September 1943, which led to Italy making a separate peace with the Allied Powers and betraying the Axis Powers. While the Allies prepared an invasion force to invade the Italian Peninsula, German troops took up key positions in Italy so that they could take over the defense of the peninsula if the Italians decided to surrender. When the Italians announced their surrender, German forces under General Kurt Student occupied Rome, although the pro-Allies government of Marshal Pietro Badoglio fled to Brindisi. Italy was soon under German occupation.
Upon Italy's surrender, the Italian Resistance was formed with the goal of resisting the German occupation of Italy. Early resistance movements at Boves in Piedmont, Bosco Martese in Abruzzo, and the Soviet and communist partisans in Venezia Giulia were crushed by rapid German reactions, and the Nazis committed their first massacre on Italian territory at Boves. The Italian Communist Party took up a vanguardist approach, officially stating that the Italian people should not passively wait for freedom from the British and the Americans, but rise up against German occupation.
On 13 October 1943, the Italian government officially declared war on Nazi Germany, but the imprisoned fascist dictator Benito Mussolini was freed from house arrest by German paratroopers and set up his Italian Social Republic based at Salo on Lake Garda in northern Italy. The Badoglio government formed the Co-belligerent Army to fight alongside the Allies against the German occupiers and their Italian collaborators, and the Italian partisans fought against the Mussolini supporters and the Wehrmacht. The partisans operated in the mountains while the fascists operated in the cities, and the German military and the Italian Decima Flottiglia MAS unit committed horrific atrocities, massacring suspected partisans. A famous episode was the "120 Days of Sodom" near Marzabotto in northern Italy in 1944. At times, both sides experienced infighting due to fears of infiltration. The brutal resistance war continued until 2 May 1945, when Heinrich von Vietinghoff ordered the surrender of all German forces on the peninsula. Many soldiers, executives, and sympathizers of the Salo Republic were subjected to show trials and executed after the war, including innocent people who were accused of collaboration by people who held private grudges against them. 12,060 fascists were executed in 1945 and 6,027 in 1946, with one historian estimating that up to 30,000 were executed.