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Why did U.S. and South Korean forces retreat to the Pusan Perimeter?

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Korean War, June–August 1950 — Korean War Map showing North Korean advances in the Korean War in June–August 1950.

U.S. and South Korean forces retreated to the Pusan Perimeter during the early stages of the Korean War because they were overwhelmed by the North Korean invasion force, which possessed significant advantages in equipment and military strength.1

Following the initial invasion on June 25, 1950, the North Korean People’s Army (KPA) utilized a formidable offensive force modeled after Soviet mechanized armies to drive south. U.S. ground forces, which were initially limited in number and hampered by inadequate weaponry and uncertain leadership, were unable to halt the advance. As these forces fell back, they were further beset by streams of refugees, which complicated military operations and increased the risk of guerrilla infiltration.1

By the summer of 1950, months of fighting had forced the United Nations Command to consolidate their remaining defensive positions into a 5,000-square-mile rectangle centered on the port of Pusan. This perimeter served as a critical logistical hub, allowing the UN to receive necessary supplies and reinforcements to eventually stabilize the front and launch a counteroffensive.12