As tensions rose in Europe, the Cold War spread to Asia. In 1945 Korea was divided into Soviet and American zones of occupation, which in 1948 became Communist North Korea and anticommunist South Korea. When the Russian-backed Communist forces of North Korea invaded South Korea in spring 1950, President Truman sent U.S. troops to lead a UN coalition force to stop what he interpreted as a coordinated Communist effort to dominate Asia.

The Korean War (1950–1953) was bitterly fought, but ended in a stalemate. The well-equipped North Koreans conquered most of the peninsula, but the South Korean, American, and UN troops rallied and drove their foes north to the Chinese border. At that point China intervened and pushed the South Koreans and Americans back south. In 1953 a fragile truce was negotiated, and the fighting stopped. Thus the United States extended its policy of containing communism to Asia, but drew back from invading Communist China and possibly provoking nuclear war.