On December 31, 1950 a force of 170,000 Chinese forces, augmented with North Korean troops, crossed the frozen Han River to attack Seoul.

General Douglas MacArthur, anticipating that UN forces wouldn’t be able to stop the Chinese advance, began making preparations for a slow retreat to the Pusan Perimeter. 

Mao Zedong, however, became convinced that Chinese forces were overextended and not in a state to push the UN out of the Korean Peninsula. He therefore decided to concentrate offensive efforts against the weak South Korean units on the 38th Parallel.

 By January 3rd, with UN forces penetrated on all sides, the UN forces retreated from Seoul.

Chinese and North Korean forces suffered ~8,500 casualties.

UN losses were ~ 800 dead, wounded, and captured. 

Source: World Atlas

After the accidental death of Lieutenant General Walton Walker on December 23, General Matthew Ridgway was assigned command of the 8th United States Army Extremely displeased with the performance of his troops, Ridgway took immediate steps to restore the morale and fighting spirit of the UN forces in Korea.

Ridgway reorganized the command structure, removed some officers, replaced division commanders who had been in action for six months with fresh leaders and ordered them to spend more time at the front lines than in their command posts in the rear.

With Ridgway leading the 8th Army, MacArthur regained confidence in the ability of UN forces to hold Korea, and abandoned preparations for evacuation of the peninsula.

Under Ridgway, with copious use of field artillery, UN tactics changed from an aggressive stance to fighting protective, delaying actions.

Chinese casualties became very high as they launched human attack waves into the coordinated artillery fire and their offensive was decisively slowed.

Source: Historynet