On 15 October, on Wake Island in the Pacific, General MacArthur and President Truman met to discuss the future of the war.
MacArthur reportedly told Truman that he was confident of early success in the North Korean offensive, and that he no longer feared Chinese intervention.
Just 10 days later, the Chinese army, which had been secretly massing at the border, made its first attack on the allies. In the days that followed, the allies' headquarters received intelligence that Chinese forces were hidden in the North Korean mountains, but this was disregarded.
The Chinese troops withdrew, and the allies interpreted these initial skirmishes as simply defensive. Undeterred, General MacArthur ordered a bold offensive on 24 November to push right up to the Yalu River, which marked the border between North Korea and north-east China.
He optimistically hoped this would finish the war and allow the troops "home by Christmas". But it was instead to mark yet another turning point in the conflict. The next day, about 180,000 Chinese "volunteers" attacked.
A shocked MacArthur told Washington: "We face an entirely new war."
He ordered a long and humiliating retreat - performed in sub-zero temperatures - which took the troops below the 38th parallel by the end of December.
As Chinese troops unleashed a renewed offensive, the allies were forced to withdraw south of Seoul in January 1951. Here, in the relatively open terrain of South Korea, the UN troops were better able to defend themselves. After a few more months of fighting, the front eventually stabilised in the area of the 38th parallel.