On June 30, 1950, American troops arrived in South Korea as part of a larger United Nations fighting force. Their immediate mission was to liberate the country, which had been invaded a few days prior by North Korea, a Soviet-supported communist regime that sought to merge the two nations into a single Marxist state. President Harry Truman's decision to send a U.S. contingent to Korea reflected the broader goal of preventing communism from spreading in Asia.
The Cold War
In World War II, the United States and the Soviet Union were part of the Allied coalition that defeated Germany, Italy and Japan. After the war, however, the two countries became the protagonists of a bitter rivalry that kept the world at the brink of war for the next four decades. Though they never fought each other directly, both the United States and the Soviet Union attempted to influence movements and uprisings in other countries against or in favor of communism, respectively. The Korean War was the first military conflict of this tense period, known the Cold War.
A Policy of "Containment"
Truman's decision to send American troops to Korea was aligned with the policy of containment that shaped the United States' approach to foreign affairs during the Cold War. Per this policy, Truman's administration and those that followed focused on preventing communism from proliferating into Western Europe and beyond. Truman saw the invasion of South Korea as merely the first move of a much larger plot by the Soviet Union and China to propagate communism throughout Asia and thus felt obligated to intervene.
Pressure From Within
Prior to the conflict in Korea, Truman had been the target of Republicans who argued that the president and his administration were slacking when it came to communism. The 1949 triumph of the Marxist revolution in China, which signed a treaty with the Soviet Union the following year, gave critics on the issue plenty of ammunition against Truman. Military intervention on behalf of South Korea therefore became a way for Truman to counter those who argued that he was not doing enough to contain communism around the world.
A Limited but Bloody War
Truman was determined to prevent a war with China or the Soviet Union. Thus, rather than strive for an unconditional victory as in American wars past, the president's goal in Korea was to restore the pro-Western government in the south. By that measure, he succeeded; by the end of the conflict in July 1953, the Korean peninsula was once again divided by its pre-war boundaries, though North Korea remained in communist hands. In the process of liberating South Korea, the United States suffered more than 50,000 casualties.