The First Battle of Maryang San (3−8 October 1951), also known as the Defensive Battle of Maliangshan (Chinese: 马良山防御战; pinyin: Mǎliáng Shān Fángyù Zhàn), was fought during the Korean War between United Nations (UN) forces—primarily Australian and British—and the Chinese communist People's Volunteer Army. The fighting occurred during a limited UN offensive by US I Corps, codenamed Operation Commando. This offensive ultimately pushed the Chinese back from the Imjin River to the Jamestown Line and destroyed elements of four Chinese armies following heavy fighting.