The Korean War was the first armed conflict of the Cold War, the first "hot war" to grow out of the Soviet-American rivalry. At the end of World War II, American and Soviet troops each occupied half of Korea, which had been ruled by Japan since the 1930s. At the war's end, the Americans controlled the southern part of the country and the Soviets the north, with the 38th parallel serving as a dividing line. When the Russians and Americans withdrew their armies in 1949, they left behind two Koreas, with each section organized according to the respective Communist or anticommunist orientation of its former occupiers. The new South Korean regime under Syngman Rhee was militarily weak and very much a dictatorship, and North Korean leader Kim Il Sung saw an opportunity to reunify the nation under his own Communist command. In March 1950, Kim went to Moscow to receive Soviet leader Josef Stalin's blessing for an invasion of South Korea, and on 24 June 1950, the North Korean army poured across the 38th parallel to invade South Korea. Communist forces quickly captured almost the entire Korean Peninsula, including the South Korean capital city of Seoul.
According to the American doctrine of containment, the United States could not allow this invasion to go unanswered. Just one year earlier, China had fallen to the Communists under Mao Zedong, and American leaders did not want to "lose" another Asian nation to the Communist orbit. President Harry Truman could not afford to look "soft on Communism," and he came to view Korea as an important test of American resolve in the Cold War. Thus the United States went to the United Nations on 27 June 1950 to seek approval for a military intervention to aid South Korea. Since the Soviet Union was boycotting the UN at the time due to the organization's refusal to recognize Communist China, the Americans faced no opposition on the Security Council. On 30 June, General Douglas MacArthur led US forces—under the aegis of the United Nations—into Korea.
Although American forces suffered several defeats in their early battles, initially managing to hold only the southern city of Pusan, MacArthur's forces surprised the North Koreans on 15 September with a daring amphibious landing at Inchon, a place far to the north, behind enemy lines. Once the Americans established their beachhead at Inchon, they quickly recaptured the South Korean capital of Seoul and began to sweep Communist forces back north, toward the 38th parallel. Success for MacArthur's forces soon led to expanded American war aims; as American troops began to dominate their North Korean foes, American policy-makers began to imagine that the war could end not with containment (pushing the Communists back across the 38th parallel) but rather with liberation (the total conquest of the North Korean regime). With President Truman's authorization, MacArthur's forces captured the North Korean capital of Pyongyang on October 19, 1950, and continued to push on toward the Yalu River, which marked the border between North Korea and China.
However, by pushing northward to the Yalu, MacArthur all but ensured that China would intervene in the war. Earlier in the fall, North Korean leader Kim Il Sung had requested aid from Moscow, and Soviet leader Josef Stalin delegated responsibility for assisting North Korea to its Communist neighbor, China. Chinese leader Mao Zedong became increasingly nervous that the Americans would not stop their advance at the Yalu River, but would push on into Chinese territory in an attempt to "liberate" China. Thus Mao sent Chinese army divisions into battle against American forces on 25 October. The Chinese assault caught MacArthur flatfooted, and Chinese and North Korean forces soon began driving the American army back to the south. By the spring of 1951, the battle line fell into stalemate at a location remarkably close to the 38th parallel. After nearly a year of intense warfare, the two sides found themselves dug in pretty much where they had started. North Korean and American troops have been staring each other down across the 38th parallel ever since. The two sides agreed to a truce half a century ago but never negotiated a formal peace treaty; technically, the Korean War continues to this day.