Since the surprise assault at Inchon in September 1950, the North Korean Army was in a northward retreat. United Nations forces had not only regained lost South Korean territory but were pushing farther up the peninsula, closing in on what seemed certain victory. But on 25 October, Chinese communist forces entered the war, pouring across the border in overwhelming numbers.
The situation rapidly deteriorated, and many U.N. units found themselves in serious trouble. Among those was the 1st Marine Division, then commanded by Major General O. P. Smith and located near a reservoir in northeast Korea known to the Koreans as “Changjin.” Because the Marines were relying on older Japanese maps that called the reservoir “Chosin,” that name found its way into the history books.