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The Battle of Pusan Perimeter was a large-scale battle between United Nations and North Korean forces lasting from August 4 to September 18, 1950. It was one of the first major engagements of the Korean War. An army of 140,000 UN troops, having been pushed to the brink of defeat, were rallied to make a final stand against the invading North Korean army, 98,000 men strong.

UN forces, having been repeatedly defeated by the advancing North Koreans, were forced back to the “Pusan Perimeter”, a 140-mile (230 km) defensive line around an area on the southeastern tip of the Korean Peninsula that included the port of Pusan. The UN troops, consisting mostly of forces from the Republic of Korea (ROK), United States and United Kingdom, mounted a last stand around the perimeter, fighting off repeated North Korean attacks for six weeks as they were engaged around the cities of Taegu, Masan, and P’ohang, and the Naktong River. The massive North Korean assaults were unsuccessful in forcing the United Nations troops back further from the perimeter, despite two major pushes in August and September.

North Korean troops, hampered by supply shortages and massive losses, continually staged attacks on UN forces in an attempt to penetrate the perimeter and collapse the line. However, the UN used the port to amass an overwhelming advantage in troops, equipment, and logistics, and its navy and air forces remained unchallenged by the North Koreans during the fight. After six weeks, the North Korean force collapsed and retreated in defeat after the UN force launched a counterattack at Inchon on September 15. The battle would be the furthest the North Korean troops would advance in the war, as subsequent fighting ground the war into a stalemate.