The Second Phase Offensive or Second Phase Campaign of the Korean War was an offensive by the Chinese People's Volunteer Army against United Nations Command 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 −30 °C 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 servicemember 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.