Bombing ISIS fighters from the air and arming moderate rebels to attack them from the ground may sound attractive. But not if we can’t really tell them apart.

Then there’s America’s second potential ground ally in Syria: Bashar al-Assad. He commands a far more unified and effective fighting force than do the “moderate” Syrian rebels. He’s less of a threat to the United States than is ISIS. And he’s even preferable morally—in a Stalin versus Hitler kind of way. Some current and former British politicians now propose allying with the Syrian regime, at least in order to ensure that if American and British warplanes enter Syria to bomb ISIS, Assad’s anti-missile systems won’t shoot them down.

But given that President Obama called on Assad to leave power three years ago and last year almost bombed him for using chemical weapons, even a tacit alliance with the Syrian dictator would make Obama’s past flip-flops look trivial. In Washington, the outcry would be massive, especially because of Syria’s close ties to Iran. Regionally, it might be worse. If relations between Washington and long-standing Sunni allies like Saudi Arabia are frayed now—in part because the U.S. hasn’t intervened against Assad strongly enough—it’s hard to imagine the impact on those relationships were the U.S. and Assad to actually join forces.