Most likely, Assad (with Russian help) will finish everyone else first.
Russia's goal is to have Assad as the winner, and they will optimize for that. Fighting against ISIS would be a waste of resources for them -- other countries will do that for them, so they can focus on the remaining Assad's enemies.
And the other big players know that. This is why the situation is so difficult to solve, although in theory it should work just as proposed (weaken ISIS by bombing, and let their enemies do the rest).
Russia's goal is to have Assad as the winner, and they will optimize for that. Fighting against ISIS would be a waste of resources for them
Um, ISIS controls large chunks of Syria and their capital, Raqqa, is a Syrian town which, I assume, Assad would like to have back.
Moreover, as far as I understand, ISIS wants to have a major battle with the Western/Christian/Crusader army by the Syrian town of Dabiq (to kick-start the Apocalypse) and will, presumably, commit all its forces to it.
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.