Today in: Hey, I know that Guy!
A former student of mine wrote a blog post: Can the U.S.-Cambodia Relationship Move On From the Lon Nol Conundrum?
In the comments section, I point out what I had learned from Sideshow: Kissinger, Nixon and the Destruction of Cambodia by William Shawcross--i.e. while the circumstances surrounding the Lon Nol coup are highly suspicious, direct CIA involvement has never been proven.  Nevertheless, the U.S. Government almost certainly could have reversed the coup by refusing to recognize Lon Nol, but neglected to do so.