Brief history of the Korean War
US troops were hurriedly sent from bases in Japan. But they and their South Korean allies fared badly in the initial confrontation with the North.
Beating a hasty retreat, they managed to hold on to a small area surrounding the port city of Busan, in the peninsula's south-eastern corner, while the US called on the United Nations Security Council for support.
The Security Council passed a resolution which called on all members to help repel the invasion. The motion was only passed because the Soviet delegate, who would have certainly vetoed it, was absent because he was boycotting Security Council meetings until China was admitted to the UN.
Fourteen UN nations - Australia, Belgium, Canada, Colombia, Ethiopia, France, Greece, the Netherlands, New Zealand, the Philippines, South Africa, Thailand, Turkey and the United Kingdom - agreed to help, committing a force of some 300,000. Most of the military support - 260,000 troops - was provided by the US, but the UK, Canada and Australia all made a substantial commitment.
While reinforcements were readied, it fell to the limited troops already in the country to hold on to the pocket of territory they still controlled. Ironically, they were helped by the fact that their defensive line was now so short - a perimeter of just 50 by 100 miles around Busan - that it was easier to defend.
North Korean's supply lines were also dangerously overstretched by their rapid advance.
It fell to the US Eighth Army commander, General Walton Walker, to rally the troops. He delivered a famous "Stand or Die" speech stressing that the force could no longer retreat.
The South Korean army and just four, ill-equipped US divisions managed to hold off the North's battalions for six weeks, but more US troops died in this battle than in any other operation during the war. They had bought time with blood.