These are perilous moments in the conflict in Ukraine. In response to the Ukrainian military's stunning gains in recent weeks, Putin is escalating. He has enacted a military mobilization within Russia and is once again threatening the use of nuclear weapons.  How seriously should we take these nuclear threats? In what scenarios and circumstances might Putin actually use a nuclear weapon. And how should the Biden administration and NATO respond if, indeed, Putin goes nuclear? We put these questions and more to Jon Wolfstal a longtime nuclear policy professional and aid to then Vice President Joe Biden who currently serves as senior advisor to Global Zero and as a board member for Bulletin of the Atomic Scientists.