The wartime partnership between the United States and the Soviet Union (USSR) was an alliance of necessity. Putting aside ideological differences and a history of mutual distrust, the two nations joined forces to combat Nazi aggression. As long as the Nazi threat existed, the alliance held, but as the war ended and attention turned to the postwar world, the allies became adversaries. The two nations did not engage directly in war, but they entered into a prolonged struggle for political, economic, and military dominance known as the Cold War.