The surprise Chinese intervention triggered Causation a retreat Escaping of UN forces back below the 38th Parallel by late December .

In these and subsequent battles Hostile_encounter , Seoul changed Change hands four times , and the last two years of fighting Hostile_encounter became Becoming a war Hostile_encounter of attrition , with the front line close to the 38th Parallel .

The UN forces Causation withdrew Escaping from North Korea to the 38th Parallel where they sought to re-establish defensive Defending positions Placing .

In the aftermath of the battle Hostile_encounter , the US Eighth Army 's heavy losses Earnings_and_losses forced Causation all UN forces to retreat Escaping from North Korea to the 38th Parallel .

China 's Chairman Mao Zedong ordered Arranging the Chinese People 's Volunteer Army to cross Motion_directional the 38th Parallel in an effort to pressure Convincing the UN forces Hostile_encounter to withdraw Escaping from South Korea .

On December 31 , 1950 , the Chinese 13th Army attacked Attack the Republic of Korea Army ( ROK ) 's 1st , 2nd , 5th and 6th Infantry Divisions along the 38th Parallel , breaching Damaging UN defenses Defending at the Imjin River , Hantan River , Gapyeong and Chuncheon in the process .