The issue with any negotiated settlement is that how do you trust Russia to hold up its end of the bargain after 2014? They've shown that international agreements mean sweet F.A. to them if it gets in the way of their goals.

The only way we get out of this in the long term to push Russia out of Ukraine, and force an unconditional Russian surrender. That means taking back the Donbas region; that means taking back Crimea.

but he should also be calling for a negotiated end to this brutal war

Bernie is smart enough to know that you can't reliably negotiate with someone as untrustworthy as Putin.

Putin has lied to the world more than enough times that nobody should trust any potential negotiation he is involved in.