Map of Chinese First Phase Campaign, October 25 – November 1, 1950
Well, if they go fast enough, maybe some of them can be home by Christmas.
— General Douglas MacArthur
In the wake of the UN forces' successful landing at Inchon, the breakout from the Pusan Perimeter and the subsequent destruction of the Korean People's Army (KPA) during September 1950, the Eighth United States Army crossed the 38th Parallel and advanced rapidly towards the Sino-Korean border. Alarmed by this development, China's Chairman Mao Zedong ordered the PVA to intervene in Korea and to launch the First Phase Campaign against the UN forces. Between October 25 and November 4, 1950, the PVA 13th Army surprised and defeated the Republic of Korea Army (ROK) II Corps and the US 1st Cavalry Division in a series of battles around Onjong and Unsan, destroying the right flank of the US Eighth Army while forcing the UN forces to retreat back to the Ch'ongch'on River. Although PVA forces were able to break through the UN line, logistics difficulties forced the PVA to withdraw on November 5, 1950.
Despite the success of the PVA First Phase Campaign, the UN planners still believed that China had not intervened in Korea on a large scale. The suddenness of the PVA withdrawal in the face of a victory further reinforced this belief. Working on the assumption that only 30,000 PVA troops could remain hidden in the hills, General Douglas MacArthur ordered the bombing of the bridges over the Yalu River in an effort to cut off PVA reinforcements. Confident that the UN air forces could detect and disrupt any troop movements across the Yalu River, MacArthur launched the Home-by-Christmas Offensive on November 24 to rout the remaining PVA and KPA forces and to end the Korean War.
Unknown to the UN planners, however, there were already 180,000 PVA troops stationed in Korea, with more reinforcements infiltrating across the border. Although the PVA was ordered to maintain a defensive posture in North Korea until Soviet weapons could arrive in the spring of 1951, its earlier successes convinced the Chinese leadership that the PVA was capable of turning the tide of UN advance. Encouraged by the fact that the UN did not know their true numbers, PVA Commander Peng Dehuai outlined the Second Phase Campaign, a counteroffensive aimed at pushing the UN forces back to a line halfway between Ch'ongch'on River and Pyongyang. As a part of a deception plan to further reinforce the weak appearance of PVA forces, Peng ordered all units to rapidly retreat north while releasing POWs along the way. With 230,000 troops at his disposal and another 150,000 heading to the Chosin Reservoir, Peng authorized the start of the Second Phase Campaign on November 22, 1950.