In 1945, Korea was split along the 38th parallel between a communist north led by Kim IL Sung, and a non-communist south led by Syngman Rhee.
But communism was growing in the Far East. In 1949, the Communists had taken power in China. The US developed the 'domino theory' - the idea that, if one country fell to communism, others would follow like a row of dominoes. Then, in 1950, a report by the American National Security Council ('NSC68') recommended that the US stop containment and start to roll back communism.
Recently, historians have shown that the Korean crisis almost led to World War Three - many US advisers wanted to use the atomic bomb.