The Second Phase Offensive (25 November – 24 December 1950) or Second Phase Campaign () of the Korean War was an offensive by the Chinese People's Volunteer Army (PVA) against United Nations Command (U.S./UN) forces, most of which were soldiers of South Korea and the United States. The two major engagements of the campaign were the Battle of the Ch'ongch'on River in the western part of North Korea and the Battle of Chosin Reservoir in the eastern part of North Korea.
Casualties were heavy on both sides. The battles were fought in temperatures as low as and casualties from frostbite may have exceeded those from battle wounds. On 23 December 1950 Lieutenant General Walton Walker died in a vehicle accident in Seoul while in command of the U.S. 8th Army. He was the highest ranked U.S. military service member killed in the Korean War.
U.S. intelligence and air reconnaissance failed to detect the large numbers of Chinese soldiers present in North Korea. Thus, the UN units, the Eighth United States Army on the west and the X Corps on the east, kicked off the "Home-by-Christmas" offensive on 24 November with "unwarranted confidence...believing that they comfortably outnumbered enemy forces." The Chinese attacks came as a surprise. The Home-by-Christmas offensive, with the objective of conquering all of North Korea and ending the war, was quickly abandoned in light of the massive Chinese assault.
The Second Phase Offensive forced all UN forces to go on the defensive and retreat. China had recaptured nearly all of North Korea by the end of the offensive.
Background
In late 1950, the United Nations military forces, led by the United States, advanced rapidly north through South Korea with only light resistance from the retreating North Korean Korean People's Army (KPA). On 1 October 1950, UN Commander General Douglas MacArthur called on North Korea to surrender. There was no response. On 3 October MacArthur announced that UN forces had crossed the border into North Korea. The UN forces proceeded northward toward the Yalu River, the boundary between North Korea and China. On 24 November a final UN offensive, informally called the "Home-by-Christmas Offensive", to complete the conquest of North Korea began. Most of North Korea was then occupied by UN forces, and the Korean War seemed to be almost over. The UN soldiers and their leaders were optimistic that the Korean War could be concluded by Christmas, 25 December.
Despite some indications that substantial numbers of Chinese soldiers were in North Korea, MacArthur played down both the estimates of numbers of Chinese soldiers in North Korea and the possibility of large-scale Chinese intervention. A U.S. military intelligence report dated 25 November estimated that the total number of Chinese soldiers in North Korea was only 70,000. That same date the PVA launched the Second Phase Offensive with 300,000 men or more.
Infiltration and opening shots
In early October 1950, after reports that UN forces were crossing the border into North Korea, Chinese leader Mao Zedong notified Soviet Union leader Joseph Stalin that "We have decided to send troops into Korea...to fight the United States...this is necessary because, should the Americans occupy the whole of Korea, the Korean revolutionary force would be completely destroyed and the American invaders would become more rampant." Mao said that China would infiltrate 12 infantry divisions into North Korea and that another 24 infantry divisions were present in northern China (Manchuria) for deployment if necessary. A Chinese infantry division when fully staffed numbered about 10,000 men. Mao also appealed for Soviet military assistance for artillery and air power. Mao said that to defeat the UN forces, China would have to have manpower "four times larger than the enemy's."
On 19 October, tens of thousands of Chinese soldiers began crossing the Yalu River, the border between China and North Korea, at night to avoid detection by UN reconnaissance aircraft. Their entry was undetected. On 25 October the first shots fired between the PVA and UN forces were at the Battle of Onjong, about south of the Yalu. This battle heralded the opening of what China called the First Phase Offensive which would last until 6 November. Peng Dehuai, the Chinese commander, ended the First Phase Offensive because his soldiers were tired and lacked food and munitions. He also said the Chinese soldiers were fearful of UN air raids and that the cold winter weather would make it "increasingly difficult to preserve the strength of our troops who have to sleep outside and sometimes in the snow." Nevertheless, Peng reported that the results of the First Phase Offensive were satisfactory.