The biggest problem with Bernie's foreign policy arguments is his constant reference to the Iraq war vote in '02. Yes, he made the right call and Hillary didn't. It's a valid point. But we get it already! He brings it up over and over again, in every issue related to foreign policy, that it comes off as his default response to anything. And he himself (seemingly) put a pin in it tonight when he said the most important thing what we can learn from the experience of going to war in Iraq... and yet then once again reminded everyone that Hillary voted for it.
Iraq is one of the biggest political blunders in history, and a large part of the reason so much of this ISIS related mess exists today. He's right to trumpet it.
I also don't think he's asleep on foreign policy, I think it's just his ideas aren't typical establishment one's. In all honesty, ISIS leadership would probably dislike his plans more than most based on tonight's responses. They want big bad evil America to bomb their nations, whilst other Muslim countries oppose each other, that way ISIS can continuously recruit more people to their cause, and paint it as some sort of Western war against Islam. If as Bernie suggested it was instead Arab nations taking more of a charge, backed by a larger coalition, it'd be a different situation and story, giving them far less fuel for their fire.