Um, ISIS controls large chunks of Syria and their capital, Raqqa, is a Syrian town which, I assume, Assad would like to have back.
Sure. Maybe I didn't express myself clearly. Assad has multiple goals. Destroying ISIS (and getting the capital back) is one of them. Destroying the rebels is another. Destroying Kurds is yet another.
If you have three goals, A, B, C, and you know that most of the world will support you with A, it makes sense to spend your resources (such as the army) on B and C first.
Scenario 1: Assad destroys ISIS first. Other countries will help him, but he will still pay a significant part of the cost. After ISIS is gone, most countries are no longer interested in helping Assad. Some of them may even object against his fight against the rebels and Kurds. Some of them may even start supporting the rebels again.
Scenario 2: Assad destroys the rebels and Kurds first. Then he looks at the rest of the world and says: "You guys are still interested in helping me destroy ISIS, right?"
Assad has multiple goals
Yes, and the first and most important goal is to survive. I don't think Assad has that much latitude in choosing which enemies to go after and which to ignore for the time being. He has been amazingly tenacious, but it's far from a foregone conclusion that he'll be the only one left standing at the end.
Here's my op-ed that uses long-term orientation, probabilistic thinking, numeracy, consider the alternative, reaching our actual goals, avoiding intuitive emotional reactions and attention bias, and other rationality techniques to suggest more rational responses to the Paris attacks and the ISIS threat. It's published in the Sunday edition of The Plain Dealer, a major newspaper (16th in the US). This is part of my broader project, Intentional Insights, of conveying rational thinking, including about politics, to a broad audience to raise the sanity waterline.