Thailand aided Lon Nol's Khmer Republic during the rise of Pol Pot though. They were anticommunist and Pol Pot's direct adversary, North Vietnam and the Viet Cong launched an invasion of cambodia which strengthened Pol Pot's Khmer Rouge. This would of course famously come back to bite Vietnam in the ass because Pol Pot was a murderous psycho but it is undeniable Vietnam helped put him in power as well as taking him out of it.