"The very first engagement of the conflict, when the North Koreans crushed South Korean defenses at the 38th parallel, demonstrated the superiority of the North Korean Army. On June 28, three days after the opening attack, a tank/infantry force leading the main North Korean thrust entered Seoul... In the face of the onslaught, the South Korean Army retreated, leaving most of its equipment behind. Whatever effectiveness it may have possessed was already lost."
Having found that all demands for a military withdrawal from the South were in vain, the United Nations Security Council convened on June 27 and requested that its member states form a military alliance and come to the aid of South Korea. President Truman committed U.S. Forces to the war that very day.
All told, Seoul would be taken and re-taken as many as four times before the war would end.