The Russian military buildup along its 1,200-mile border with Ukraine has renewed concerns Russia will again invade its neighbor. It is clear the U.S. and NATO will not defend Ukraine. Without outside help, Ukraine stands little chance of resisting an attack.

With deterrence basically off the table, invasion then turns on Moscow’s whims and on whether the United States and European powers can prevent an attack with a diplomatic push that answers Russia’s major demands.

What political outcome will satisfy both the United States and Russia? What reforms to Europe’s security order would help stave off Ukraine-Russia conflict and future crises?