The Khmer Rouge is an interesting example, as it was a left-wing totalitarian government that the USA supported[0], albeit often at arms length. It was part of the geopolitics at the time, as the USA was aligned with the PRC and the Khmer Rouge against the Soviet bloc. A lot of it was also a petty vendetta against Vietnam, as arming the Khmer Rouge was a way to get back at Vietnamese Communists for winning the Vietnam War.

[0] https://en.wikipedia.org/wiki/Allegations_of_United_States_s...