Not as ISIS. We've trained an supported "freedom fighters" in Syria who turned and eliminated less extreme rebels. We've destabilized Syria and given ISIS a place to grow. Now, they're taking weapons we left behind in Iraq, and they move freely across what used to be the border with Syria.
We should've been on Assad's side. We've seen it all over the middle east. There's a reason these countries are run by dictators with varying degrees of ruthlessness. They keep their populations in line.
Assad also supports terrorists. We took the right side, the FSA which is staffed mostly be Syrian military defectors as opposed to political or religious thugs.
You will see in the link I provided that the FSA and its allies have been fighting Assad and ISIS for some time.