> I see no evidence that the CIA directly gave weapons to Al-Nusra or other Al-Qaeda affiliates. And zero evidence of any direct funding or support of ISIS
You stick a very key word there, directly. In fact the US is who funded and armed Osama bin Laden and his followers jihad against the secular government of Afghanistan in the late 1970s and 1980s - but the CIA did it via the Pakistan ISI. The Maktab al-Khidamat actually had connections to organizations all over the USA.
The US press says Cambodia was taken over in 1975 by communists led by Pol Pot, and in a de facto sense that is somewhat true but in a strict de jure sense it is not. Sihanouk, the Cambodian king who had been ousted by a CIA-backed coup by Lon Nol was who officially ran the government. He was in a political, governmental and military coalition with the communists. The US government and press saw his role as a figurehead and it's not that inaccurate. OK, the Cambodian communists are ousted in the 1979 and suddenly the US is looking to oppose Vietnamese communist influence in Cambodia. So who do they fight to keep the UN seat of? Who does US intelligence fund and arm? The political/military coalition of Sihanouk and the Cambodian communists. Which in the 1970s was called the Khmer Rouge. But suddenly things aren't happening directly, and the fig leaf of Sihanouk and other minor figures and groups becomes much bigger. This was reported in the New York Times at the time,
and Nightline went even further, taking a camera crew to Cambodia and showing exactly what was going on - that the US was arming the "Khmer Rouge" rebels whom it had been accusing of genocide a few years earlier.
If a direct link is what is needed then you're not going to find much, as they're not that dumb and incompetent.