Carter and his national security advisor Zbignew Brzezinski were more intent to normalize political and business ties with China, the major patron of the Khmer Rouge regime, than worrying about Cambodian peasants. Note that diplomatic relations between China and Uncle Sam were established on January 1, 1979, six days before the fall of Khmer Rouge regime.
The first two paragraphs of a commentary by John Pilger, in the Covert Action Quarterly (Fall 1997) with the caption, ‘The Long Secret Alliance: Uncle Sam and Pol Pot’, offers the following details:
“The US not only helped create conditions that brought Cambodia’s Khmer Rouge to power in 1975, but actively supported the genocidal force, politically and financially.