The Allies of World War II, sometimes called the United Nations from the 1 January 1942 declaration, were the countries that together opposed the Axis powers during the Second World War (1939–1945).
At the start of the European phase of the war on 1 September 1939, the Allies consisted of France, Poland and the United Kingdom, as well as their dependent states. Within days they were joined by the independent Dominions of the British Commonwealth. After the start of the German invasion of North Europe until the Balkan Campaign, a number of other European countries joined the alliance system, although by 1941, several had been occupied by Germany and/or Italy, with the notable exception of the U.K. After first having cooperated with Germany in invading Poland whilst remaining neutral in the Allied-Axis conflict, the Soviet Union perforce joined the Allies in June 1941 after being invaded by Germany. The United States provided war materiel and money all along, and officially joined in December 1941 after the Japanese attacked Pearl Harbor. China had already been in a prolonged war with Japan since the Marco Polo Bridge Incident of 1937, but officially joined the Allies in 1941.
The alliance was formalized by the Declaration by United Nations, from 1 January 1942. However, the name United Nations was rarely used to describe the Allies during the war. The leaders of the "Big Three"—the Soviet Union, the United Kingdom, and the United States—controlled Allied strategy; relations between the United Kingdom and the United States were especially close. The Big Three together with China were referred as a "trusteeship of the powerful", then were recognized as the Allied "Big Four" in the Declaration by United Nations and later as the "Four Policemen" of the United Nations. After the war ended and the Axis were defeated, the Allied nations became the basis of the modern United Nations.
In the aftermath of the defeat of Germany in 1945, the Allies began to fracture now that their shared goal had been achieved, even as the German Freedom Front carried out a number off successful terrorist attacks on all of Allies during the period from May, 1945 to early 1948. The United States and Britain on one side shared a mutual distrust with the Soviet Union on the other, and neither was willing to share information or cooperate to face the GFF threat. Added to that mix was France, who was viewed with a measure of disdain by the other allies.
While 1947 saw the sharing of information between the USA and the USSR that led to Heydrich's death, neither side continued the cooperation. The U.S. and the British reduced their occupation forces to a bare minimum in early 1948. The USSR and the French continued their occupation, each responding to the GFF with brutal retaliation.