It is absolutely essential for the Western coalition to get the Syrian government’s approval for any direct action against IS on Syrian territory. The Assad regime will give it gladly, since IS is its primary enemy. The time for squeamishness over taking to Assad and refusals to countenance a Syria with his regime in charge has long since gone in a plethora of misreadings and misjudgements. Things have moved on and it is the general consensus that this extremist combination of various branches of Sunni militancy is an evil to be stopped.
The web of so called allies is singular and likely to be ineffective. Saudi Arabia and the Gulf States were the original source of IS funds. Turkey is rumoured to buy IS oil. The majority of IS fighters are Saudi. The alternative combination of the West, Iran, Syria, and Russia supported by a general mobilization of Shias and Kurds wherever they are domiciled, would guarantee a swift end to this menace. That would then allow negotiations with moderate Sunnis, who have been sucked in to supporting IS, so that they could have some territory to call their own.
This requires the West to overcome its differences with both Russia and Iran, neither of which pose a real threat and both of which have everything to gain economically from re-integrating sanction free with the global economy. To achieve this requires a broad strategic vision based upon the realities which are, rather than ideological aspirations which are not. In other words no more tribal politics and more statesmanship.
The alternative is to walk into the trap set by IS with its gruesome and cruel beheadings. Their idea is to lure the West into another conflict with opaque loyalties, unrealistic hopes and flawed strategy. This will lead to another Afghanistan, Iraq or Libya. Military intervention followed by insurgency in a self replicating cycle which could last a hundred years.