Why did NATO and the Warsaw Pact each form?

In 1949, the prospect of further Communist expansion prompted the United States and 11 other Western nations to form the North Atlantic Treaty Organization (NATO). The Soviet Union and its affiliated Communist nations in Eastern Europe founded a rival alliance, the Warsaw Pact, in 1955. The alignment of nearly every European nation into one of the two opposing camps formalized the political division of the European continent that had taken place since World War II (1939-45). This alignment provided the framework for the military standoff that continued throughout the Cold War (1945-91).

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