In the Korean War (1950-53) a U.S.-dominated United Nations coalition came to the aid of South Korea in responding to an invasion by North Korea, which was aided by the USSR and allied with Communist China; the war ended in a military stalemate and the restoration of the political status quo. Concurrently, the United States was assuming increasing leadership of the Western nations against what were perceived as the expansionist intentions of its former ally, the USSR. As this cold war heated up, it brought the United States into a military confrontation with Communist forces in Korea.

(1998 Grolier Multimedia Encyclopedia)

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