World War II: The Allies commence Operation Shingle, an assault on Anzio and Nettuno, Italy.

The Battle of Anzio was a battle of the Italian Campaign of World War II that took place from January 22, 1944 (beginning with the Allied amphibious landing known as Operation Shingle) to June 5, 1944 (ending with the capture of Rome). The operation was opposed by German forces in the area of Anzio and Nettuno.The operation was initially commanded by Major General John P. Lucas, of the U.S. Army, commanding U.S. VI Corps with the intention being to outflank German forces at the Winter Line and enable an attack on Rome.

The success of an amphibious landing at that location, in a basin consisting substantially of reclaimed marshland and surrounded by mountains, depended on the element of surprise and the swiftness with which the invaders could build up strength and move inland relative to the reaction time and strength of the defenders. Any delay could result in the occupation of the mountains by the defenders and the consequent entrapment of the invaders. Lieutenant General Mark W. Clark, commander of the U.S. Fifth Army, understood that risk, but he did not pass on his appreciation of the situation to his subordinate Lucas, who preferred to take time to entrench against an expected counterattack. The initial landing achieved complete surprise with no opposition and a jeep patrol even made it as far as the outskirts of Rome. However, Lucas, who had little confidence in the operation as planned, failed to capitalize on the element of surprise and delayed his advance until he judged his position was sufficiently consolidated and he had sufficient strength.

While Lucas consolidated, Field Marshal Albert Kesselring, the German commander in the Italian theatre, moved every unit he could spare into a defensive ring around the beachhead. His artillery units had a clear view of every Allied position. The Germans also stopped the drainage pumps and flooded the reclaimed marsh with salt water, planning to entrap the Allies and destroy them by epidemic. For weeks a rain of shells fell on the beach, the marsh, the harbour, and on anything else observable from the hills, with little distinction between forward and rear positions.

After a month of heavy but inconclusive fighting, Lucas was relieved and sent home. His replacement was Major General Lucian Truscott, who had previously commanded the U.S. 3rd Infantry Division. The Allies broke out in May. But, instead of striking inland to cut lines of communication of the German Tenth Army's units fighting at Monte Cassino, Truscott, on Clark's orders, reluctantly turned his forces north-west towards Rome, which was captured on June 4, 1944. As a result, the forces of the German Tenth Army fighting at Cassino were able to withdraw and rejoin the rest of Kesselring's forces north of Rome, regroup, and make a fighting withdrawal to his next major prepared defensive position on the Gothic Line.

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.