After World War II, the Soviet Union and the United States were the two most powerful countries left in the world. The Cold War was a period of tension between the two countries over ways of life. The two countries tried to get other countries on their side. The Soviet Union tried to get countries to become Communist and the United States tried to stop them from being Communist. American and Soviet soldiers never fought in battles, but they fought indirectly in the Korean War (1950s) and the Vietnam War (1950s–1970s).