On June 25, 1950, North Korea launched a surprise attack on South Korea across the 38th parallel. With lightning speed, the North Korean army overran South Korean and US positions, driving down the peninsula. After only about a month of bloody fighting, South Korea and its UN allies found themselves pinned down in a small corner of land around the city of Pusan (now spelled Busan), on the southeast coast of the peninsula.
Throughout August and the first half of September, 1950, the allies fought desperately with their backs against the sea. The war seemed to have reached a stalemate, with South Korea at an extreme disadvantage.
On September 15, however, US Marines made a surprise counter-attack well behind North Korean lines, at the coastal city of Incheon in northwestern South Korea. The Invasion of Incheon allowed the allied troops to break out of the Pusan Perimeter, and begin to push the North Koreans back into their own country, turning the tide of the Korean War.