Hate to break it to you but the Invasion of North Korea was a complete success. The problem was Mac so scared the Chinese that the Soviets convinced them they had no chose but to attack the UN force.
China was not prepared or equipped for a war with the UN. They were only one year off securing the entire Chinese mainland. They had no money to buy weapons and ammunition. All they had was a large very experienced Army. It lacked vehicles weapons and ammunition.
The lack of rifles was so bad that part of every unit was unarmed with orders to pick up the rifle of their fallen comrades. They did not have very many crew served weapons like Machine guns and mortars either. They relied on foot and horse/mule for supply and movement. Their Air force was non existent.
Numbers was their only strong point and as would be borne out in the coming years a refusal to quit.
America had the mission completed after the Inchon landing led to the NK being cut off and surrounded and defeated. UN (US ) Forces recaptured the SK capital and captured the NK capital of Pong Yang. North Korea was defeated. Meanwhile China was reluctant to enter the conflict and warned Truman that it would support N.K. if the Yalu was approached by UN . MacArthur disregarded the warning. You should know this Gunny, MacArthur never spent a single night in Korea. He relied on General Willoughby to push Marines beyond their supply lines and faster than O.P. Smith thought was prudent. The Army walked into the biggest ambush in history but the Marines were able to avoid being overrun. The invasion of NK was a complete disaster. It turned a victory in less than a year into a slogging truce at the cost of 35,000 American lives in three years.