edit: at the same time, in the same conflict.
I remember reading a few years ago about a conflict in (I want to say) 1980’s Latin America where this occurred; however I can’t find anything about this with current online search tools. Did I hallucinate this or did this actually, and if so what groups?
The closest I can think of: I think there was a crazy time when the People’s Republic of China and, to a lesser extent, the US were together(!) backing “Democratic Kampuchea” (i.e. the Khmer frickin’ Rouge) against the invading (USSR-allied) Socialist Republic of Vietnam and its proxy, the People’s Republic of Kampuchea.
So, the US, softly, backing a bunch of far-left lunatics against a less-far-left bunch backed indirectly by the Soviets.
From Wikipedia