In Foreign Policy magazine, Noah Bonsey writes:

U.S. officials publicly acknowledge that the Syrian regime’s behavior — indeed its very nature — is a primary factor fueling the jihadis’ rise and that Syrian President Bashar al-Assad’s forces continue to kill far more civilians (and rebels) than the Islamic State does. They also recognize that the role of mainstream rebels will be essential in reversing jihadi gains.

The U.S.-led coalition’s strikes have enabled the regime to reallocate assets to face mainstream rebels, whose defeat remains the regime’s top priority. Since strikes against the Islamic State began, regime forces have gained ground against mainstream rebels on key fronts in Hama province and in Aleppo city; in the case of the latter, they have done so against the very same rebel groups that are confronting the Islamic State in the nearby northern countryside.