Historians generally date the Cold War from the end of World War II to the collapse of the Soviet Union in 1991. It refers to the intense rivalry for world domination between the United States and its allies on one hand and the Soviet Union and other communist countries on the other. While there were no direct military conflicts between the two superpowers, there were several armed clashes with military support by one side or the other or both. In the early 1950s, the Korean War pitted the South Korea, United States and others under the flag of the United Nations against North Korea and Communist China.
The answer to this question is: To fight the spread of Communism.
Remember the Containment Policy!!! The policy was created to find a way to contain communism in other countries. (example: Vietnam, North Korea, China, and The Soviet Union or Russia)
