The Second Phase Offensive was an offensive by the Chinese People's Volunteer Army (PVA) against UN forces. 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 (−22 °F) and casualties from frostbite may have exceeded those from battle wounds. 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.