As President Biden travels to Brussels for an emergency NATO summit, The Circus examines Russia's threats of using nuclear weapons. Jennifer Palmieri speaks to Pentagon Press Secretary John Kirby about the U.S. response to Putin's nuclear saber-rattling; John Heilemann games out war simulations with one of the world's foremost experts on nuclear weapons, Zia Mian; and Alex Wagner assesses Putin's motives with former Russian Foreign Minister Andrei Kozyrev.