Forget principles and morality. Forget, or try to forget, the quarter-million deaths for which President Bashar Assad is responsible, directly or indirectly, since choosing to respond with violence to a peaceful uprising of the Syrian people. Set aside the fact that Assad's forces have caused 10 to 15 times more civilian deaths thus far than the Islamic State militant group, whose horrific execution videos have overshadowed the Syrian dictator's invisible massacres. But even if you can purge all of this from your thoughts, a policy for Syria that posits Assad as an "alternative" to the Islamic State is simply not viable.
Assad, after all, literally unleashed the Islamic State's current savagery: in May 2011, he released hundreds of Islamic radicals from prison, quickly supplying the infant group with fighters and leaders. He then methodically shelled positions held by moderate rebels, while no less methodically sparing the Islamic State's stronghold in Raqqa. And then, in mid-2014, he allowed Iraqi elements of the Islamic State to find sanctuary in eastern Syria.
In other words, Assad created the monster that he is now pretending to fight. Is all that not a little much for a potential ally? Can working with Assad possibly provide a sound basis for what is supposed to be a common effort?