Micah Zenko picked up  on something important in recent Senate testimony from Secretary Hagel and Gen. Dempsey:
MCCAIN: How will we help them—will we repel Bashar Assad’s air assets that will be attacking them?
HAGEL: Any attack on those that we have trained and who are supporting us, we will help ‘em [bold mine-DL].
In short, because the U.S. is foolishly arming and training some of the Syrian opposition, the administration believes that it is also committed to defending them from regime attacks. Note that Hagel said that “any attack” on those opposition forces would trigger U.S. “help.” So the administration appears to have committed the U.S. to go to war against the Syrian government if the regime attacks U.S. proxies, which the regime almost certainly will continue to do. That would put our forces in the absurd and dangerous position of fighting both ISIS and the Syrian regime at the same time. The legal justifications for the ISIS war are laughably weak, but there is no legal justification in U.S. or international law for what the administration is proposing here. Just in case we might hope that this was just an off-the-cuff, meaningless answer from Hagel, his position was later confirmed  by the Pentagon.