A great friend and ally of the United States.
So was Suharto. So were the Guatemalan death squads who committed the anti-Maya genocide. So were the Khmer Rouge. So are al-Qaeda. That means nothing.
The Khmer Rouge was an avowed enemy of America from the beginning, as was Al Qaeda. Please check your facts next time. The only member of the Khmer Rouge the U. S. ever supported was Sihanouk, who was never a real leader of the Khmer Rouge and had defected from the U. S. democratic side only after a coup that objectively established a democracy.
Not true, and indeed was he ever actually a KR member?