The Battle of San Pietro Infine (commonly referred to as the "Battle of San Pietro") was a major engagement from 8–17 December 1943, in the Italian Campaign of World War II involving Allied Forces attacking from the south against heavily fortified positions of the German "Winter Line" in and around the town of San Pietro Infine, just south of Monte Cassino about halfway between Naples and Rome. The eventual Allied victory in the battle was crucial in the ultimate drive to the north to liberate Rome. The battle is also remembered as the first in which the troops of the Italian Royal Army (Regio Esercito) fought as co-belligerents of the Allies following the armistice with Italy.