It's very easy to vote against a war when you're some backbench nobody from a state nobody has ever heard of (much like it was very easy for Barack Obama to vote against the debt ceiling as an Illinois junior senator,but not when he was in office). What about the decisions now? On every foreign policy issue of any possible relevance to 2016 - the pivot to Asia, Russia's adventures in Ukraine, NATO expansion, the CIA and the creep of military intelligence, militarised drones, ISIS and the rise of Salafism, the Iran-Saudi proxy war, nuclear weaponry, Korea, Israel/Palestine - I have seen no real distinction between him and any other generic democrat. The only thing I've seen is protectionism, because Vietnamese people deserve unemployment and poor working rights, I guess.
I mean, not that I support, say, the sudden abolition of NATO; but it is a bit disingenuous of Sanders to dress himself up as the most left-wing thing since nationalised sliced bread factories, and then parade a foreign policy essentially identical to Obama's and Clinton's.
I think what's disingenuous is Clinton decrying the Iraq War to be a huge mistake as if she wasn't one of the ones who made it. Not sure if the implication is Sanders voted against Iraq solely for political posturing or Clinton should be excused for her vote because voting against an idiotic war would be politically hard, but it doesn't matter since either point is ridiculous. The Iraq War wasn't simply one of several foreign policy decisions from over a decade ago. To get back to the OP, it was a catastrophic mistake that made everything worse, paved the way for ISIS and continues to create instability. Clinton got it wrong. Sanders got it right. Their judgement matters. And yes, if Trump is nominee, there's an argument that Sanders is a stronger nominee for the Democrats. Not only because of Iraq, but also on trade, independence from big donors and voter enthusiasm.
