Mainly because she supports a no fly zone in Syria — something that would need to be backed up by some U.S. military muscle to be done effectively.
Bernie Sanders, on the other hand, is firmly against a no-fly zone.
"And what I believe Chuck, very much, is that the most powerful military on Earth, the United States of America, that our government should do everything we can to resolve international conflict in a way that does not require war," Sanders said on NBC's Meet The Press.
Martin O'Malley has come out on Sanders' side, advocating against a no-fly zone.
"I believe that a no-fly zone right now, it's not advisable," O'Malley said on CNN.
As for ISIS — all of the candidates agree that ISIS must go, and that a coalition of nations — not the U.S. on its own — is the best way to do it.