Using their vast military resources in the Mediterranean, the Allied authorities resolved to invade Italy in January 1943. British Prime Minister Winston Churchill (1874–1965) labeled Italy the "soft underbelly of Europe." Goals included removing Italy as a combatant from WWII, securing the Mediterranean Sea, and pressuring Germany to pull forces away from the Russian front and northern France, where the Allies planned to land via the English Channel in Normandy. The decision to launch an assault on Italy was not made lightly. American leaders were hesitant to transfer any resources away from Normandy, despite persistent requests by Soviet Premier Joseph Stalin (1879–1953) for the other Allies to relieve his soldiers battling Germany in the east by attempting an Allied invasion from the west. However, Italy wasn't far from the North African theater, where the Allies had a surplus of troops that might be redeployed. Churchill felt that, so long as the Allies kept the initiative, these men could fight their way rather quickly up the Italian peninsula, which would help the Normandy campaign. The majority agreed with his point of view.

Airborne and amphibious landings on the southern coast of Sicily began on July 10, 1943, as part of Operation Husky, the code name for the invasion of Sicily. As the Allies had planned, the fascist regime in Italy quickly fell into contempt after the Allied invasion. Benito Mussolini (1883–1945), Italy's prime minister, was ousted and jailed on July 24, 1943. Under Marshal Pietro Badoglio (1871–1956), who had opposed Italy's allegiance to Nazi Germany, a new temporary government was established, and he immediately began covert conversations with the Allies about an armistice. The Allies expected one more confrontation when they marched on Messina on August 17, 1943, but instead found that 100,000 German and Italian troops had escaped to the Italian mainland. Even though the war for Sicily was over, the Allies' success was weakened by their inability to catch the retreating Axis armies. In the meantime, the German high command sent sixteen additional divisions to the Italian mainland. Adolf Hitler (1889–1945), leader of Germany, did not want the Allies to set up air bases in Italy and thereby endanger the country's southern cities and key oil supplies in Romania. Field Marshal Albert Kesselring (1885–1966), who commanded an army group in southern Italy, was given orders to make the Allies pay dearly for every inch of their progress.

On September 9, 1943, American troops arrived in Salerno, Italy, but were nearly driven back into the Tyrrhenian Sea by the German army, which had quickly taken over Italy's defense. For four months, the Allied force was paralyzed by the stubborn resistance of German troops entrenched in the high Apennine Mountains at Cassino. In response to the failure of a planned rapid advance inland at Anzio, Churchill was heard to say, "I had hoped we were hurling a wildcat onto the shore, but all we got was a stranded whale." Muddy, undulating hills, flooded rivers, and washed-out roads hindered the Allies and helped the Germans defend areas where the mountains had receded. The ingenious German commander Kesselring had his troops construct many lines of defense across the small Italian peninsula. The Gustav Line, the southernmost, was situated directly behind Monte Cassino. The Allies had air superiority over all of Italy, but it took four bloody battles spanning several months to finally break through the Gustav Line and secure control of Monte Cassino. When the Allies broke out of Anzio and Cassino in May 1944, they led Kesselring's major forces straight into a trap. A contentious and poorly understood decision was made by U.S. General Mark Clark (1896–1984), who disobeyed instructions to cut off the retreating German soldiers at Cassino and instead headed northwest to conquer Rome. His choice likely wasted a chance for a speedy end to the arduous Italian Campaign by allowing a substantial German army to flee.

General Clark's Fifth U.S. Army entered Rome on June 4, 1944, but the Italian Campaign was put on hold while preparations were made for the D-Day landings in Normandy on June 6. The Allies pulled six divisions out of Italy to reinforce their landings in southern France. Fall's persistent downpours slowed Allied progress in Italy. The Allies' high command decided that continuing the Italian offensive was less important than tying down as many German units as possible for the duration of the war. By the time the German forces in Italy capitulated on May 2, 1945, two days after the fall of Berlin, Allied troops had advanced into the Po Valley in northern Italy. Despite initial confidence following the Allied victory in North Africa in 1943, the Allied battle in Italy proved to be a bloody, drawn-out, and expensive failure. There were a total of 59,000 American casualties at Anzio. Many men were exhausted from fighting in places like Monte Cassino. When the fascist state in Italy fell and a new government supportive of the Allies took its place, the war for Italy devolved into a protracted bloodbath between tenacious Allied forces and obstinate German forces. The war in Europe was the final straw that put an end to it. At that point, the death toll among the American and British forces fighting in Italy had risen to over 300,000. About 434,000 Germans lost their lives.