Why did the Chinese Ninth Army Group become combat-ineffective after the Chosin Reservoir campaign?

The Chinese Ninth Army Group became combat-ineffective following the Chosin Reservoir campaign due to a combination of severe environmental conditions and intense military pressure.

During the withdrawal of the U.S. X Corps, the 1st Marine Division and other Allied forces inflicted heavy casualties on the Chinese divisions through relentless firepower. By the time the campaign concluded in December 1950, the Ninth Army Group had been effectively ruined by three primary factors:

• Combat Losses: The 1st Marine Division rendered seven Chinese divisions ineffective during the fighting at Yudam-ni, Hagaru-ri, and the subsequent movement to the coast.1
• Environmental Exposure: The troops suffered extensively from the extreme winter weather, with temperatures dropping as low as −34 °C (−30 °F). Many soldiers were incapacitated or killed by the cold.1
• Starvation: Logistical failures left the Chinese forces struggling with a lack of supplies, leading to widespread starvation.1

These combined factors resulted in an estimated 40,000 to 80,000 casualties, forcing the Ninth Army Group to withdraw from the front lines until March 1951.1