The correct response is: Cold War competition began between the U.S. and the Soviet Union right after the end of World War II, when its alliance unraveled.
The origins of Cold War competition can be traced back to the ideological differences and growing tensions that emerged after World War II. The United States and the Soviet Union had united against a common enemy, Nazi Germany, but their alliance was fragile and based on mutual necessity rather than shared values. Once the war concluded, their differing political ideologies—capitalism and democracy in the U.S. versus communism in the Soviet Union—led to suspicions and competition for influence in Europe and beyond. Events like the division of Germany, the establishment of communist governments in Eastern Europe, and differing approaches to the reconstruction of Europe further intensified these tensions, marking the beginning of the Cold War era.