In recent decades the United States has led international efforts to halt the proliferation of weapons of mass destruction. But many of President Trump’s statements on nuclear weapons suggest a coming shift in US policy. In questioning America’s security guarantees to its allies, criticizing the Iran nuclear deal, and calling for the strengthening of America’s nuclear capabilities, critics charge that the President is undermining global nonproliferation efforts. Could America’s new posture spark a renewed nuclear arms race? Or is an expanded and modernized nuclear deterrent critical to American security in an age of geopolitical instability and renewed great power rivalry?