As the United States comes to grips with how to deal with the large ISIS presence in Syria, the majority view, as expressed succinctly by Ryan Crocker, ex-Ambassador to (inter alia) Baghdad, on the Charlie Rose Show on 3 September, we should strike at ISIS in Syria, but in no way in partnership with the regime of Bashar al-Assad, however simpler it would be to coordinate our actions with the government in Damascus. Though the U.S. has its own blemished record supporting dictators, Bashar al-Assad is a special case, as I will point out below.
Surprisingly, there are some voices, on both sides of the Atlantic, who are advocating, as a possible option, teaming up with the Assad regime to attack ISIS. I would note that negotiations expert Robert H. Mnookin (Bargaining with the Devil: When to Negotiate, When to Fight, New York: Simon and Schuster, 2010), while acknowledging that as a principle one should not refuse to negotiate with one's enemies, points out that in certain circumstances this may not be wise.
It seems to me that certain circumstances apply in the Syrian situation today, where the memories of atrocities, which are still ongoing, are altogether fresh: 191,000 persons killed since the beginning of the insurgency in March 2011, according to a UN report of 22 August 2014; 6.4 million internally displaced persons during that period; and 2.9 million persons displaced outside Syria. (USAID figures). Not to speak of the regime's gas attacks against its own population. Assad is in fact almost certainly a war criminal.
Another reason for keeping our distance from Bashar al-Assad, is that, while the U.S. has every reason to retaliate against the barbarous Sunni jihadists of ISIS, aligning itself with the Shia-related regime in Damascus would send a bad signal to the Sunni Muslim world which constitutes 85 percent of Islam. In other words the U.S. must proceed very carefully: while seeking to demolish ISIS, not getting crosswise with the Sunni world.