On June 25, 1950, Soviet-backed North Korea invaded U.S.-backed South Korea, sending its forces across the agreed boundary at the thirty-eighth parallel in an attempt to bring the entire peninsula under its control. In response, President Harry S. Truman approved U.S. military action to help South Korea. Truman and others believed what was at stake was not simply U.S. interests in Asia but also the confidence of U.S. allies everywhere in U.S. willingness to stand up to communist aggression. The military action was in accordance with UN Security Council Resolution 83—advanced by the United States—which authorized the use of force to “repel the armed attack.”
After several weeks of fighting, U.S. and South Korean forces were defending an area called the Pusan perimeter, a zone of southeastern South Korea around the port of Pusan, against North Korea’s southward advance. General Douglas MacArthur, who was commanding U.S. and allied forces as part of the UN mission, was also making plans for a landing at Inchon, near the South Korean capital Seoul.
A successful Inchon landing would mean the prospect of swiftly liberating South Korea. Truman faced a weighty decision. Should the United States simply restore the status quo ante bellum, pushing North Korea’s military back to the thirty-eighth parallel? Or should U.S. forces, leading UN forces in what Truman called a “police action,” advance into North Korea and try to unify the peninsula? On September 1, 1950, Truman pledged U.S. help for the Koreans “to be free, independent, and united.” But there was no consensus among administration officials, members of Congress, and other leaders on how far U.S. involvement should extend.
A U.S. move into North Korea would therefore bring the risk of war with the Soviet Union, China, or both. However, it would also carry the possibility of a unified peninsula, at peace, under leadership friendly to the United States. Merely restoring the status quo would reduce the risk of war with the communist powers. It would also, however, preserve a hostile North Korean leadership and leave the divided peninsula as an open wound, and potential battleground, in the intensifying Cold War.