Under the Obama administration the CIA was funneling billions of dollars worth of military equipment -- ranging from small arms and ammo, to vehicles and even anti-tank missiles -- into the hands of Syrian militant groups[1][2].

These groups specifically include ISIS.

It's hard to think that the Syrian civil war would have been so bloody or lasted as long as it did without USA involvement.

[1] https://www.globalresearch.ca/the-mystery-of-isis-toyota-arm...

[2] https://www.businessinsider.com.au/isis-weapons-cia-missile-...

Syria had a civil war before significant US involvement, but between being bordered by a completely destabilized Iraq, and the US campaign against Assad means that US involvement made the situation so much worse.

But it certainly intentionally fanned the flames and added a lot of oil after that.