After the drive to the Yalu and the impending complete defeat of North Korea, the Chinese entered the war with huge numbers of troops since they could not allow their client state to fall. They were possibly emboldened in this since no one had dropped a couple dozen a-bombs on their border area.
However, MacArthur and his staff believed that the Chinese would not attack in any numbers and did not pay attention to signs that they would, right up until they did.
The Chinese side of the Yalu remained a safe haven and dividing line across which we could not attack for fear of enlarging the war, so it settled into a back and forth stalemate - a tactic repeated in some ways in Vietnam with the safe havens of Laos and Cambodia.
