Assad and ISIS

Strategy is strategy, be it business or political. That said, I'd like some reader opinion on the situation in Syria.

The Situation:

So no one likes Assad. He's been accused of everything from supporting terrorism to terrorizing and even using poison gas on his own people. However, he's not a complete idiot and has curtailed some of his actions in the wake of international pressure. But the international community has been reticent to actually provide substantial support to these rebels.

Enter ISIS:

Thanks to western ambivalence and the power vacuum it has created, ISIS has seized much of Syrian rebel-held territory and is now poised, according to Turkey, to seize a key Kurdish town in Syria. Syrian forces are standing by idly.

Here's where I see Assad coming out on top. If he waits long enough, ISIS will defeat the rebels for him. At this point, the international community will be forced to help Assad seize back rebel-held territory from ISIS.

Imagine that, the US and its allies helping Assad recapture rebel territory. Question is, what strategy would be in the United States' best interest?