Chongchon River
The Chinese struck in the west, along the Chongchon River, and completely overran several South Korean divisions and successfully landed a heavy blow to the flank of the remaining UN forces. The ensuing defeat of the U.S. Eighth Army resulted in the longest retreat of any American military unit in history. In the east, at the Battle of Chosin Reservoir, a 30,000-man unit from the U.S. 7th Infantry Division and U.S. Marine Corps was also unprepared for the Chinese tactics and was soon surrounded, though they eventually managed to escape the encirclement, after inflicting heavy casualties on six Chinese divisions.