Early in September, T. R. Fehrenbach explains (in This Kind of War), Chinese forces began the long march from the south, where they had been deployed against Taiwan, to the mountains along the Yalu.
Chou En-lai told the Indian ambassador, “If the United States, or United Nations forces cross the 38th parallel, the Chinese People’s Republic will send troops to aid the People’s Republic of Korea. We shall not take this action, however, if only South Korean troops cross the border.” This message was passed along but was seen as diplomatic blackmail:
The Chinese had at least 38 divisions in 9 field armies garrisoned in Manchuria north of the Yalu. Of these, 24 divisions were disposed along the border in position to intervene. This estimate of CCF strength was reasonably accurate.
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Willoughby’s analysis described the open failure of the North Koreans to rebuild their forces, and suggested that this indicated the CCF and Soviets had decided against further investment in a losing cause.
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FECOM was at best a collective agency, not an evaluative one for matters of international policy; if Washington permitted FECOM both to collect and to make decisions, then whatever happened the fault was Washington’s.
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And above all else, it was the terrain and a complete failure of Intelligence that brought disaster. Marching north, the U.N. trumpeted to the world its composition, its battle plan, and even the hour of its execution.
Without effort, the enemy knew everything there was to know about the U.N. forces.
The U.N., in turn, never knew the enemy existed — until it was much too late.