The People's Volunteer Army ambushed the South Korean II Corps at the Battle of Onjong, and elsewhere engaged the 1st Infantry Division at the Battle of Unsan, marking China's entry into the Korean War.

The People's Volunteer Army (PVA), officially the Chinese People's Volunteers (CPV), was the armed expeditionary forces deployed by the People's Republic of China during the Korean War. Although all units in the PVA were actually transferred from the People's Liberation Army (PLA) under the orders of Chairman Mao Zedong, the PVA was separately constituted in order to prevent an official war with the United States. The PVA entered Korea on 19 October 1950 and completely withdrew by October 1958. The nominal commander and political commissar of the PVA was Peng Dehuai before the ceasefire agreement in 1953, although both Chen Geng and Deng Hua served as the acting commander and commissar after April 1952 following Peng's illness. The initial units in the PVA included 38th, 39th, 40th, 42nd, 50th, 66th Corps; totalling 250,000 men. About 3 million Chinese civilian and military personnel had served in Korea throughout the war.

The II Corps is a corps of the Republic of Korea Army and it was first activated during the Korean War.

The Battle of Onjong, also known as the Battle of Wenjing, was one of the first engagements between Chinese and South Korean forces during the Korean War. It took place around Onjong in present-day North Korea from 25 to 29 October 1950. As the main focus of the Chinese First Phase Offensive, the People's Volunteer Army (PVA) 40th Corps conducted a series of ambushes against the Republic of Korea Army (ROK) II Corps, effectively destroying the right flank of the United States Eighth Army while stopping the UN advance north toward the Yalu River.

The 1st Infantry Division is a military formation of the Republic of Korea Army's I Corps. The division was established on 1947 under the command of Colonel Kim Suk-won.