Final answer:
The US and GB entered into an alliance with the Soviet Union during World War II primarily to defeat Germany. The alliance was also influenced by the Soviet Union's significant sacrifices in the war. However, the differences in ideologies and postwar visions led to tensions and the start of the Cold War.
Step-by-step explanation:
The United States and Great Britain entered into an alliance with the Soviet Union during World War II mainly due to the shared goal of defeating Germany, which was a common enemy. The war required the three countries to work together and make compromises. Additionally, the Soviet Union's significant sacrifices in terms of casualties played a role in the decision to form an alliance. However, after the war, the ideological differences and competing visions for the postwar world between the US and the Soviet Union eventually led to tensions and the beginning of the Cold War.