The case for working with Assad conveniently ignores that it was other rebel groups, not Assad, who dealt the Islamic State its most serious setbacks when a coalition of anti-regime forces took heavy casualties to push it away from much of northern Syria earlier this year. These successes came because these rebel brigades could count on local support and intelligence and had legitimacy as Syrian nationalist forces. Unfortunately, the already outgunned rebels, because they lacked fresh supplies of weapons and ammunition and because they manned another front against Assad forces, could not sustain these advances. And when the Islamic States seized massive arsenals in Iraq, the balance of power shifted dramatically.

The more honest advocates of a partnership with Assad seem to bet on the very unlikely prospect of an alliance between the regime and the rebel mainstream groups against the Islamic State. Yet, such an alliance will not emerge simply because the Islamic State is unpopular among the rebels.

First, the idea of Assad as a lesser evil has little traction among Syria’s Sunnis. This is only too understandable: It is the House of Assad, not the Islamic State, that has tormented them for more than four decades. Many of these Syrians wonder why the jihadis’ beheadings warrant intervention while Assad’s far more numerous atrocities do not.

Second, what popular hostility there is toward the Islamic State stems largely from the fact that until this summer, it had refrained from attacking regime forces and focused its military efforts on fighting the rebels, validating in Syrian eyes the exaggerated notions that the jihadis were either controlled or in cahoots with Damascus. Yet, the tacit alliance between the regime and the Islamic State has now crumbled as the recent capture of bases around Raqqa has started to make the regime nervous. What’s more, the last few months in Iraq have shown that the Islamic State need not be genuinely popular to secure de facto support from the Sunni mainstream: It is enough for it to appear as the most potent challenge to a sectarian, anti-Sunni central state. And as in Iraq so it is in Syria.

Might the threat from the Islamic State induce the Assad regime to be more inclusive, thereby providing mainstream rebels with an incentive to take its side? Not a chance: The idea that it is possible to engineer a power-sharing arrangement in Damascus with the support of Iran and Russia is out of touch with reality. Since Assad’s “reelection” in June, the regime has widened its crackdown on the National Coordination Committee, one of the few legal “opposition” groups — and a Russian-Iranian favorite. Moscow’s protégé and loyal oppositionist Qadri Jamil only lasted a year in government before being pushed out for being too independent-minded. Some in Iran eager to appear constructive have floated the idea of finding a ‘Syrian Karzai,’ only to come back empty-handed. The ruling clan knows too well that its hold on power is too fragile to survive any genuine power-sharing. For all intents and purposes, Assad is the regime and the regime is Assad.

Another option might be for the regime to shower former rebel groups with patronage, following the model of the “Sunni Awakening” in Iraq. Again, this is wishful thinking: Syria’s oil wealth is negligible compared to Iraq’s, and the Islamic State now controls most of it. The Syrian regime already faces great difficulties in providing adequate patronage to loyalist forces, as shown by the latter’s resort to looting, ransoming and abusive tolling. The small number of rebel units that could be included into the regime’s patronage network would inevitably be targeted by their disenfranchised former brothers in arms, and the Islamic State would take advantage of the discord. Any alliance between the regime and former rebels would be too fragile and narrowly based to offer any credible resistance to the jihadis, and pro-regime Sunni fighters would probably end up doing what their Iraqi counterparts did in June 2014: lay down arms and flee, or more ominously, join the Islamic State altogether.

The good news it that Western leaders have so far firmly rejected any cooperation with Assad. Barack Obama, Francois Hollande and David Cameron have all dismissed such ideas on moral and strategic grounds. The head of the U.S. National Counter-Terrorism Center Matthew Olsen made this reasoning clear last week: “As long as Assad is in that position — a ruler with no legitimacy in his own country — we have seen that Syria is a magnet for extremism.” Moreover, essential partners such as Turkey and the Gulf states are less likely to support any strategy predicated on collaboration with Assad.

Over time, though, the Western official position may weaken: Assad will look for ways to entangle Western countries by reaching out through third parties, voluntarily sharing information or conducting raids on Islamic State strongholds to show himself useful. It would be tragic if tactical expediency rather than strategic soundness were to drive Western policy in Syria. After three years of wishful and negligent policymaking, it is time for a clear-headed approach, one that sees Assad as the arsonist fireman he has always been.