We can do it this way instead. The fastest way to defeat ISIS is to arm and support the strongest force in the region. In Syria, that's Assad. Yes, the dictatorial thug we were going to bomb last year. The thing is this. We don't even have to do much of anything. The Russians already support him because he granted them a Naval Base in Syria. They need him to win.
So instead of a three or four way civil war in which Assad will eventually win anyway, we get out of the way, and offer our support to Assad. Finding some weaker group, a group that is more politically palatable, if less effective, and arming and training them won't work for shit. That smaller group will decide that Assad is the real enemy, and much like the last group to join or at least come to terms with ISIS, they would be marginalized or wiped out.
So if you are serious about destroying the group ISIS in Syria, you work with the strongest faction that will not join them, or come to terms with them. That's Assad. Anything else is half assed hopeful nonsense that is not fighting to win, it's fighting not to lose, which is completely different.
Yes, Assad is a bad man. Yes, Assad is a dictator. Yes, Assad is an asshole who used chemical weapons on people. Assad will win the Civil war, the Russians won't allow any other outcome. So feeding weapons to the other groups that aren't ISIS or Assad does nothing except exacerbate the situation in which ISIS flourishes. More people see ISIS as a real legitimate power, with a real chance of winning. The war drags on for another decade, and eventually Assad wins. The moderate group we were helping blames us for not helping them enough for them to win, so allies become enemies for the next round of bomb a terrorist group bingo.
Assad is a madman, a brutal dictatorial thug. But he has the best chance of winning, and he will probably win anyway. We might as well help the guy with the best chances, and perhaps get one nation there that doesn't detest us and instead, only dislikes us a bit.
But we'll never do that, because we can't forget we wanted to bomb him last year. So instead we'll pretend that the tiny group of semi-moderate rebels has a real chance. Eventually we'll either have to let them be defeated, or we'll commit more troops to make sure they aren't. Either way, we lose.
A few bombs isn't going to do the job. A handful of rebels aren't going to stop fighting Assad and focus on ISIS. They will join ISIS, or at least come to some sort of agreement to work out their differences later, after Assad is destroyed. We look incompetent, and that's always a good way for our party to win elections.