I was stationed in Korea while in the Army. I studied the history of the Korean War quite a bit. The entry of the Chinese Communist Forces(CCF) in November of 1950, when the Allies thought the war was won and all would be home by Christmas, turned the tide of the war. Tens of thousands of Chinese slammed into the GIs. The war would rage for another 2.5 years.
In this case (having been a combat engineer) I recreated the 2nd Infantry Division's 2nd Engineers battle against the CCF. To learn more about the Battle of the Chongchon River, check out this link: Battle of the Chongchon River