Answer:
Because Communist China entered the war on North Korea's side.
Explanation:
However, as the Americans moved north toward the Yalu River, the border between North Korea and Communist China, their advance was halted when China entered the fray that fall. In late November, a massive force of Chinese troops sent the Allies into retreat. In early January 1951, the Communists recaptured Seoul, only to have the Allies reoccupy it in March.
Mark me brainliest if I helped:D
