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. The war
In 1950, after getting the support of Russia and China, Kim IL Sung invaded South Korea. The North Korean People's Army (NKPA) easily defeated the Republic of Korea's army (the ROKs). By September, the NKPA had conquered almost the whole of South Korea. The USA went to the United Nations and got them to send troops to defend South Korea. The Russians couldn't veto the idea because they were boycotting the UN at the time. In September, UN troops, led by the US General MacArthur, landed in Korea and drove the NKPA back. By October, the UN forces had almost conquered all of North Korea. In November 1950, Chinese People's Volunteers attacked and drove the Americans back. They recaptured North Korea, and advanced into South Korea.
The Americans landed more troops and drove the Chinese back to the 38th parallel, where Truman ordered General MacArthur to stop and sacked him when he disagreed. The war went on as border clashes until 1953 when America's new president, Eisenhower, offered peace, but threatened to use the atomic bomb if China did not accept the offer. Recently, historians have shown that the Korean crisis almost led to a third world war - many US advisers wanted to use the atomic bomb.