The american take over of Communist North Korea was cut short. overjoyed americans forgot they were in the middle of a battle, only to be met by a surprise attack from Chinese troops. They had to retreat eventually.
The Chinese decided to enter the Korean War on November 25-26, 1950. This was an enormous shock to the American forces, as well as to the United Nations. The United States, and in particular U.S. Gen. Douglas MacArthur, believed that the Chinese would not openly enter the war. When China did provide support to North Korea, the effect was catastrophic on the American forces, leading to a great expansion of the conflict.
