In late October, 1950, the South Koreans started capturing enemy soldiers who didn’t speak Korean, T. R. Fehrenbach explains (in This Kind of War), and assumed these were Chinese volunteers:
Chinese troops were deliberately misschooled on their own order of battle, so that, captured, they might tell weird tales. There were clashes between Americans and Chinese “volunteers” in odd places — obviously to draw American attention from where the Chinese planned to strike.
[...]
Air patrolling over the mountains revealed what it had always revealed — nothing. Only heavy, aggressive ground patrolling into the hills could have revealed that the main bodies of two massive Chinese army groups lurked in those deep valleys and forlorn villages, and this action the U.N. never attempted.
In the frightful terrain such patrolling was dangerous. It could not be supported by wheels, and where wheels could not go, neither could sizable units of Americans. And in such horrendous terrain a vast army could be — and was — hidden in a very small area, observing perfect camouflage discipline, waiting.
[...]
As the month progressed, however, FECOM came more and more to the conclusion that there were Chinese troops in Korea. Their numbers were placed at between 40,000 and 70,000. Whether “volunteers,” as the Chinese Government claimed, or otherwise, the big question remained as to what they were doing in Korea.
There seemed to be three possibilities, all of which were suggested:
- The Chinese had come over in limited fashion to help the NKPA hold a base south of the Yalu;
- They had entered as a show of force to bluff the U.N. into halting south of the river;
- At the worst, they were a screening force to cover the advance of the main Chinese armies.
No one, either in FECOM or the two commands in Korea, suggested that the CCF were already in Korea in massive force.