Manchuria

The brainwashing happens in Manchuria because that is where the novel's Chinese and Soviet handlers could plausibly meet.

Richard Condon's 1959 novel has an American platoon captured in Korea and taken across the border into Manchuria, where a soldier is conditioned to kill on command. The 1962 film adaptation dropped out of circulation for two decades, which did its reputation no harm. The phrase broke loose from the story around 1970 and is now applied to any politician suspected of serving another country.