I went to the Killing Fields last April and I remember hearing there that the U.S. government actually supported Pol Pot, at least indirectly, following his government's exile via Vietnamese intervention. To what extent is this true, and why?
Other related questions: What was the international response when news of the atrocities leaked through? Why did the Vietnamese invade in the first place? Was it for humanitarian reasons? Why did China support Pol Pot against the Vietnamese?
To what extent is this true
So there was plenty of support in the West for Pol-Pot after his government was ousted.
The reasons are foggy for me, but Cambodia's UN seat was held by the Khmer Rouge for some time after their ouster.
In addition, Ta Mok claims that the SAS ran some training camps for the Khmer Rouge, and that foreign aid for Cambodian refugees in Thailand was diverted to the Khmer Rouge with the knowledge of the Americans and British.
Later the U.S. opposed a Vietnamese withdrawal from Cambodia that came with conditions that would keep the Khmer Rouge from seizing power.
So plenty of support there.
Why did the Vietnamese invade in the first place
A series border disputes that spiraled out of control. Many Khmer regard large portions of South Vietnam as a part of Cambodia. So there was no love lost between the heavily nationalist Khmer Rouge and the also heavily nationalist Vietnamese.
I really like this guy for further reading on basically anything to do with modern Cambodia.