On 9 February 1945, a force of Allied Bristol Beaufighter aircraft suffered many losses during an attack on the German destroyer Z33 and its escorting vessels; the operation was called Black Friday by the survivors. The German ships were sheltering in a strong defensive position in Frde Fjord, Norway, forcing the Allied aircraft to attack through massed anti-aircraft fire (FlaK).

The Beaufighters and their escort of Mustang Mk III fighters from 65 Squadron RAF were intercepted by twelve Focke-Wulf Fw 190s of Jagdgeschwader 5 (Fighter Wing 5) of the Luftwaffe. The Allies damaged at least two of the German ships for the loss of seven Beaufighters shot down by FlaK. Two Beaufighters and a Mustang were shot down by the Fw 190s and four or five of the German aircraft were shot down by the Allied aircraft, including that of the ace Rudi Linz.

The decision to attack Z33 and her escorts, rather than a nearby group of merchant ships, followed instructions from the Admiralty to RAF Coastal Command. The losses led to merchant ships being made the priority over destroyers and small warships. Another squadron of P-51 Mustangs was assigned to protect Allied aircraft operating near Norway from German fighters.

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.