In the aftermath of the defeat of Germany in 1945, the Allies began to fracture now that their shared goal had been achieved, despite the rise of the German Freedom Front under the leadership of Reinhard Heydrich. Despite 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. Neither was willing to share information or cooperate to face the GFF threat. Added to that mix were the French, 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.
When the Race's Conquest Fleet arrived on Earth in 1942, World War II was suspended and the Allies were forced to reach a ceasefire with the Axis. The US, the USSR, and Britain joined with Germany and Japan to form the Big Five, and all other Allies either resisted the Race alongside the Big Five or became Race colonies, excepting those which had been absorbed into enemy states before the Conquest Fleet's arrival.