On June 25, 1950, North Korean People's Army forces crossed the 38th Parallel with 135,000 troops and 150 T-34 tanks. The Republic of Korea Army had no anti-tank weapons capable of stopping T-34s. US forces, understrength after postwar demobilization, suffered costly defeats arriving piecemeal from Japan. By early August, UN forces held a rectangular perimeter in the southeastern corner of Korea—approximately 140 miles long, 50 miles wide—anchored on the port of Pusan. General Walton Walker made his position explicit: there would be no more retreats.

The Defense

Walker's defense was a masterpiece of economy and improvisation. His primary advantage was interior lines—the perimeter was small enough that he could shift reserves by truck faster than NKPA forces could reposition around the outside. His primary problem was that the NKPA probed the entire perimeter simultaneously. Walker maintained a mobile reserve that he rushed to each crisis as it developed, plugging gaps with whatever units were available regardless of service or nationality.

Logistics as Decisive Factor

The fundamental asymmetry was logistical. UN forces were supplied through the port of Pusan—an inexhaustible pipeline from Japan and the continental United States. NKPA supply lines stretched 400 kilometers from the Yalu River, subject to constant UN air attack, and logistically inadequate for sustained offensive operations. By August, NKPA divisions committed to battle at 40–60% establishment, with critically short ammunition and fuel.

The Breakout

When MacArthur's Inchon landing on September 15 threatened to cut off the entire NKPA, Walker launched a general breakout. NKPA forces, already logistically exhausted, collapsed rapidly. The army that had nearly overrun South Korea in June disintegrated in September, survivors fleeing north in small groups.