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.