World War II divided Korea into a Communist, northern half and an American-occupied southern half, divided at the 38th parallel. The Korean War (1950-1953) began when the North Korean Communist army crossed the 38th Parallel and invaded non-Communist South Korea. As Kim Il-sung's North Korean army, armed with Soviet tanks, quickly overran South Korea, the United States came to South Korea's aid. General Douglas MacArthur, who had been overseeing the post-WWII occupation of Japan, commanded the US forces which now began to hold off the North Koreans at Pusan, at the southernmost tip of Korea. Although Korea was not strategically essential to the United States, the political environment at this stage of the Cold War was such that policymakers did not want to appear "soft on Communism." Nominally, the US intervened as part of a "police action" run by a UN (United Nations) international peace- keeping force; in actuality, the UN was simply being manipulated by US and NATO anti-Communist interests.
With the US, UN, and South Korean (ROK) forces pinned against the sea at Pusan, MacArthur orchestrated a daring amphibious assault on Inchon, a port on the western coast of Korea. Having made this landing, MacArthur crushed the North Korean army in a pincer movement and recaptured Seoul, the capital of South Korea. Instead of being satisfied with his rapid reconquest of South Korea, MacArthur crossed the 38TH Parallel and pursued the North Korean army all the way to the northernmost provinces of North Korea. Afraid that the US was interested in taking North Korea as a base for operations against Manchuria, the People's Republic of China secretly sent an army across the Yalu River. This Chinese army attacked the US/UN/ROK forces. Only after the appointment of Lt. General Matthew Ridgway as commander of ground forces did American morale improve and the initiative begin to swing against the Chinese Communists.
Although President Truman hoped to end the war quickly and pressed MacArthur to be more tactful, the brilliant strategist went against presidential orders and continued spouting incendiary lines about his hopes to reunify Korea. After gaining the support of the Joint Chiefs of Staff (JCS), Truman relieved MacArthur of command. The move was extremely unpopular in America; MacArthur was perceived as a popular war hero. Only the support of the JCS saved Truman from impeachment after the firing.
Ridgway took MacArthur's command and held off the Communists with strong fortifications and entrenchments just north of the 38TH Parallel, sending occasional offensives against the Iron Triangle, the Communists staging area for attacks into South Korea. Peace negotiations dragged on at Kaesong, then moved and continued to drag at Panmunjom through 1951 and 1952. The US tried using strategic bombing to intimidate the Communists into negotiating a peace treaty, but they wouldn't budge, particularly on the issue of POW (Prisoner of War) repatriation. Neither side wanted to appear weak, and so the talks went on, occasionally breaking down for months. Only after Eisenhower, who was a war hero and was unafraid of Republican criticism (since he himself was a Republican), became President, could the US make substantial concessions to the Communists. In 1953 a peace treaty was signed at Panmunjom that ended the Korean War, returning Korea to a divided status essentially the same as before the war. Neither the war nor its outcome did much to lessen the era's Cold War tension.