From Herman's article at the Commentary site;
After 1970, Communist North Vietnam steadily armed and trained the Khmer Rouge to do battle against the pro-American government in Phnom Penh. At the time, antiwar activists in the United States were perfectly aware of this, and wholly approved: here was the Vietcong’s Cambodian twin, another group of Asian revolutionaries struggling to liberate their country from the yoke of neo-colonialism.
Those people were not anti-war then, and they are not anti-war now. That is the biggest lie of all. They are simply anti-American and will do whatever they can to support America's enemies.
The only thing that Herman neglects in his piece, and I wish that he hadn't, is the absolute complicity or the United States Department of State in all of this. THEY were the big players and had a GREAT deal to do with facilitating the overthrow of Diem and all that followed.