Operation Avalanche was the codename for the Allied landings near the port of Salerno, executed on 9 September 1943, part of the Allied invasion of Italy during World War II. The Italians withdrew from the war the day before the invasion, but the Allies landed in an area defended by German troops. Planned under the name Top Hat, it was supported by the deception plan Operation Boardman.

The landings were carried out by the U.S. Fifth Army, under Lieutenant General Mark W. Clark. It comprised the U.S. VI Corps, the British X Corps, and the U.S. 82nd Airborne Division, a total of about nine divisions. Its primary objectives were to seize the port of Naples to ensure resupply, and to cut across to the east coast, trapping the Axis troops further south.

In order to draw troops away from the landing ground, Operation Baytown was mounted. This was a landing by the British Eighth Army, under General Sir Bernard Montgomery, in Calabria in the 'toe' of Italy, on 3 September. Simultaneous sea landings were made by the British 1st Airborne Division at the port of Taranto (Operation Slapstick). General Montgomery had predicted Baytown would be a waste of effort because it assumed the Germans would give battle in Calabria; if they failed to do so, the diversion would not work. He was proved correct. After Baytown, the Eighth Army marched 300 miles (480 km) north to the Salerno area against no opposition other than engineer obstacles.

The Salerno landings were carried out without previous naval or aerial bombardment in order to achieve surprise. Surprise was not achieved. As the first wave approached the shore at Paestum a loudspeaker from the landing area proclaimed in English, "Come on in and give up. We have you covered." The troops attacked nonetheless.

The Germans had established artillery and machine-gun posts and scattered tanks through the landing zones which made progress difficult, but the beach areas were captured. Around 07:00 a concerted counterattack was made by the 16th Panzer Division. It caused heavy casualties but was beaten off. Both the British and the Americans made slow progress, and still had a 10 miles (16 km) gap between them at the end of day one. They linked up by the end of day two and occupied 3545 miles (5672 km) of coastline to a depth of 67 miles (9.711.3 km).

Over 1214 September the Germans organized a concerted counterattack by six divisions of motorized troops, hoping to throw the Salerno beachhead into the sea before it could link with the British Eighth Army. Heavy casualties were inflicted, as the Allied troops were too thinly spread to be able to resist concentrated attacks. The outermost troops were therefore withdrawn in order to reduce the perimeter. The new perimeter was held with the assistance of naval and aerial support, although the German attacks reached almost to the beaches in places.

The Allies, later known formally as the United Nations, were an international military coalition formed during the Second World War (1939–1945) to oppose the Axis powers, led by Nazi Germany, Imperial Japan, and Fascist Italy. Its principal members by 1941 were the United Kingdom, United States, Soviet Union, and China.

Membership in the Allies varied during the course of the war. When the conflict broke out on 1 September 1939, the Allied coalition consisted of the United Kingdom, France, and Poland, as well as their respective dependencies, such as British India. They were soon joined by the independent dominions of the British Commonwealth: Canada, Australia, New Zealand and South Africa. Consequently, the initial alliance largely resembled that of the First World War.

As Axis forces began invading northern Europe and the Balkans, the Allies added the Netherlands, Belgium, Norway, Greece, and Yugoslavia. The Soviet Union, which initially had a nonaggression pact with Germany and participated in its invasion of Poland, joined the Allies in June 1941 after Operation Barbarossa. The United States, while providing some materiel support to European Allies since September 1940, remained formally neutral until the Japanese bombing of Pearl Harbor in December 1941, after which it declared war and officially joined the Allies. China had already been at war with Japan since 1937, but formally joined the Allies in December 1941.

The Allies were led by the so-called "Big Three"—the United Kingdom, the Soviet Union, and the United States—which were the principal contributors of manpower, resources, and strategy, each playing a key role in achieving victory. A series of conferences between Allied leaders, diplomats, and military officials gradually shaped the makeup of the alliance, the direction of the war, and ultimately the postwar international order. Relations between the United Kingdom and the United States were especially close, with their bilateral Atlantic Charter forming the groundwork of the alliance.

The Allies became a formalized group upon the Declaration by United Nations on 1 January 1942, which was signed by 26 nations around the world; these ranged from small nations far removed from the war, to governments in exile from the Axis occupation. The Declaration officially recognized the Big Three and China as the "Four Powers", acknowledging their central role in prosecuting the war; they were also referred to as the "trusteeship of the powerful", and later as the "Four Policemen" of the United Nations. Many more countries joined through to the final days of the war, including colonies and former Axis nations.

After the war ended, the Allies, and the Declaration that bound them, would become the basis of the modern United Nations; one enduring legacy of the alliance is the permanent membership of the U.N. Security Council, which is made up exclusively of the principal Allied powers that won the war.