Final answer:
Great Britain and the United States allied with the Soviet Union during World War II due to a shared enemy in Germany, despite having different visions for postwar Europe. Strategic military necessities and the high Soviet casualties made the alliance essential for the defeat of the Axis powers. After the war, ideological differences between the US and the USSR concerning the future of Europe's political landscape led to the emergence of the Cold War.
Step-by-step explanation:
During World War II, Great Britain and the United States wanted to work with the Soviet Union because they shared a common enemy in Germany. The war effort necessitated collaboration and compromise, despite the differing postwar visions for Europe held by these nations. In the gameplay of power politics, the Big Three met in Tehran in 1943 to discuss war strategies and plan the postwar world order, wherein the Soviet Union made significant demands due to the heavy casualties and sacrifices they had suffered.
Both the United States and Great Britain recognized the strategic importance of the Soviet Union in defeating Germany, which is why they acceded to some of Stalin's requests, such as the territory seized from Poland in 1939 remaining under Soviet control. Additionally, the United States assumed they would require Soviet support for the defeat of Japan until 1945. Moreover, economic and political ideologies differed drastically, with the United States favoring democracy and capitalism, while the Soviet Union sought to create a buffer zone of friendly communist states for protection against future invasions.
As the war concluded, the alliance began to dissolve due to these incompatible visions for the future of Europe and the world, leading to the Cold War. The United States was motivated to expand its influence and protect democratic governments globally, while the Soviet Union aimed to retain control over Eastern Europe and promote communism as a means to secure itself from potential threats.