When China entered the war in November 1950, the UN forces (mainly American and Korean) were strung out from pursuing the retreating North Korean army and scattered about the country. The North Korean army which had chased US and Republic of Korea (ROK) forces south, almost the whole length of the peninsula, and bottled them up in a tiny bridgehead around the port of Pusan, had been themselves routed following an amphibious landing of US Marines deep in their rear at Inchon in September. From this time on, they had been pursued hotly north, beyond Seoul, across the 38th Parallel, and deep into North Korea itself. North Korea's army was broken, and the UN forces seemed poised to reunite Korea under Syngman Rhee. 8th Army moved into northwest Korea, while X Corps drove northeast toward Manchuria. The two formations were widely separated and unable to support each other. The Chinese intervention, and the shock it produced on scattered American forces, must be understood in this context.

The initial wave of attacks began October 25 and lasted through early November. They took UN forces almost totally by surprise. MacArthur had said that there would be no Chinese intervention; the time for that had passed. Then an ROK corps was virtually annihilated at Onjong, in the northeast, when it was ambushed by two Chinese divisions. The US 2nd Infantry Division was battered. A few days later, a Chinese assault shattered an American cavalry regiment and a Korean infantry regiment at Unsan, in the northwest. The Americans began to draw back, stung.

But then, the attacks ceased. The Chinese retreated north, into the mountains. The Americans misinterpreted this. They believed that it signified a lack of will to decisively intervene on China's part. The advance was continued. This proved disastrous. More than 300,000 fresh Chinese troops struck suddenly in late November. Regimental Combat Team 31, more than 3000 strong, was virtually destroyed on the east side of the Chosin Reservoir after a harrowing breakout attempt through multiple Chinese divisions. To the west, the Marine 1st Division was forced to fight for its life, suffered huge casualties, and made a legendary retreat. X Corps was mauled, and 8th Army was in rapid retreat, one that would not stop until UN forces reached positions in South Korea. The lines would shift again and again, eventually stabilizing by 1952 or so, but this would mark the last time that Chinese forces achieved a dramatic strategic victory over UN forces.

source? they fought to a standstill, didn't they, with the stalemate lines actually being slightly north of the pre-war border? "stalemate" does not equal "defeat"

Roughly two-thirds of the troops that fought for the South in the Korean War were South Korean, though it's true that the Americans were very much better equipped and trained at the start.

The end result was a line shoved south towards Seoul in the west, but more northwards in the east, and an approximately stable line being held. If the setbacks in mid 1950 and the end of 1950 be considered the "horrible defeat", we might on the same grounds ask "How was North Korea so horribly defeated during the Korean War?" due to the events of late 1950.

Can someone address "voluntary force"? Starting with 6 organized armies and totalling by the end two millionish men, how "voluntary" were they?