The Battle of Monte Cassino (also known as the Battle for Rome and the Battle for Cassino) was a costly series of four assaults by the Allies against the Winter Line in Italy held by Axis forces during the Italian Campaign of World War II. The intention was a breakthrough to Rome.

At the beginning of 1944, the western half of the Winter Line was being anchored by Germans holding the Rapido-Gari, Liri and Garigliano valleys and some of the surrounding peaks and ridges. Together, these features formed the Gustav Line. Monte Cassino, a historic hilltop abbey founded in AD 529 by Benedict of Nursia, dominated the nearby town of Cassino and the entrances to the Liri and Rapido valleys. Lying in a protected historic zone, it had been left unoccupied by the Germans, although they manned some positions set into the steep slopes below the abbey's walls.

Repeated pinpoint artillery attacks on Allied assault troops caused their leaders to conclude the abbey was being used by the Germans as an observation post, at the very least. Fears escalated along with casualties and in spite of a lack of clear evidence, it was marked for destruction. On 15 February American bombers dropped 1,400 tons of high explosives, creating widespread damage. The raid failed to achieve its objective, as German paratroopers then occupied the rubble and established excellent defensive positions amid the ruins.

Between 17 January and 18 May, Monte Cassino and the Gustav defences were assaulted four times by Allied troops. On 16 May, soldiers from the Polish II Corps launched one of the final assaults on the German defensive position as part of a twenty-division assault along a twenty-mile front. On 18 May, a Polish flag followed by the British Union Jack were raised over the ruins. Following this Allied victory, the German Senger Line collapsed on 25 May. The German defenders were finally driven from their positions, but at a high cost. The capture of Monte Cassino resulted in 55,000 Allied casualties, with German losses estimated at around 20,000 killed and wounded.

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.