For the U.S. to fight ISIS in Syria it would need Syria's acceptance of this. Some are saying that when we forced Assad to get rid of Chemical weapons it was like allying with him in some ways.
However, in this situation, Assad actually helped strategically create ISIS exactly as seen in this political cartoon out of France:
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So, to ally with Assad for the U.S. would be tactical to try to wipe out ISIS. But, Strategically there is a level in which in some ways ISIS is aligned with Assad. There is sort of an agreement between thieves that they have so to speak. So, we cannot jeapordize our fighting personnel to any kind of trust between the U.S. and Assad because he might sell us out to ISIS. Because as long as ISIS exists Assad might stay in power in Syria too.
Also, if he sold them out they are in some ways a lot scarier than the U.S. Because he doesn't want his and his whole family's heads on ISIS spikes one day he is much more likely to sell out the U.S. than ISIS in the end.
So, once again a U.S. alliance between Syria and the U.S. could only be tactical but never strategic. But, selling out the U.S. could be strategic for Assad's survival as a middle eastern leader long term.