The Battle of the Pusan Perimeter was the desperate defensive phase of the Korean War in which American and Republic of Korea forces halted their retreat in the southeastern corner of the peninsula. The line around Busan was a narrow foothold under sustained attack, but holding it prevented North Korean forces from overrunning South Korea and gave United Nations forces time to reinforce their position.

Retreat and the decision to hold

After fighting at Taejon, exhausted groups of American soldiers made their way through hostile country toward safety. The retreat continued through the steep passes of the Sobaek Mountains to a defensive position on high ground behind the Nakdong River. In seventeen days of fighting, American and Republic of Korea forces had fallen back seventy miles. The withdrawal was a grave setback, but it also bought time for men and supplies to reach the port of Busan.

On July 29, General Walton Walker, the tactical United Nations commander in the field, ordered his troops to make a stand: there would be no further retreat, and Busan must not fall. His order contemplated fighting until help arrived, even if that meant fighting to the death. The troops, however, did not share the public-facing comparisons to earlier defeats that accompanied the announcement. Some regarded the speech as unnecessary; the prospect of another Dunkirk or Bataan had not been on their minds.

The perimeter

By the first week of August, American and Republic of Korea forces had established the final defensive line, later known as the Battle of the Pusan Perimeter. It formed a thinly held arc 130 miles long around the port. On a map, the line appeared little more than a toehold on the peninsula; on the ground, the fighting remained fierce. The monsoon rains had ended, leaving troops to contend with heat and dust. Dust became so thick that supply trucks kept their headlights on at midday.

The North Korean advance had also come at considerable cost. North Korean casualties were heavier than the Americans had expected, and their supply lines had become greatly overextended. United Nations forces held control of the sea and air, while American tanks, artillery, and fresh troops were arriving rapidly at Busan. The defenders’ position remained precarious, but the balance of operational pressures was beginning to change.

Command and political authority

MacArthur, in overall command of United Nations forces, was trading space for time: the retreat allowed forces and supplies to build up at Busan. Despite the suffering and humiliation of the withdrawal, the American and Republic of Korea units had prevented the North Korean advance from sweeping across South Korea. Truman later described their performance as a heroic rearguard action.

The campaign also raised a question about the President’s authority in an emergency. Averell Harriman urged Truman to ask Congress for a war resolution while the country still supported the action in Korea. Dean Acheson opposed the request, arguing that it was unnecessary and could cause harm; he advised Truman to rely on his constitutional authority as Commander in Chief. Truman agreed, reasoning that appealing to Congress at that moment could make it harder for future presidents to respond to emergencies. The choice did not alter the fighting at the perimeter, but it established the constitutional basis on which his administration was conducting the intervention.

A precarious line begins to stabilize

The perimeter did not immediately turn the campaign into a secure advance. It was a thin defensive arc, and fighting continued along it. Yet the North Korean forces’ extended supply lines and mounting losses stood in contrast to the growing flow of United Nations matériel and personnel through Busan. At a briefing on August 12, Omar N. Bradley characterized the situation as fluid but improving. The immediate retreat was over; the perimeter had held long enough for the military situation to begin shifting.