The Battle of Unsan (Hangul: 운산전투; hanja: 雲山戰鬪; RR: Unsan jeontu; MR: Unsan chŏnt'u), also known as the Battle of Yunshan (Chinese: 云山战斗; pinyin: Yún Shān Zhàn Dòu), was a series of engagements of the Korean War that took place from 25 October to 4 November 1950 near Unsan, North Pyongan province in present-day North Korea. As part of the Chinese First Phase Campaign, the People's Republic of China's People's Volunteer Army made repeated attacks against the Republic of Korea 1st Infantry Division near Unsan beginning on 25 October, in an attempt to take advancing United Nations forces by surprise. In an accidental first encounter with the United States military during the Korean War, the Chinese 39th Corps attacked the unprepared US 8th Cavalry Regiment in Unsan on 1 November, resulting in one of the most devastating US losses of the Korean War.