Russia might use nuclear weapons to achieve its goals in the war in Ukraine—a risk that has only grown as Russian forces confront Ukrainian counteroffensives. Such nuclear use could advance the Kremlin’s military aims, undermine U.S. interests globally, and set off a humanitarian catastrophe unseen since 1945. To deter such a potential disaster, the United States should issue public, deliberately vague threats of serious consequences for any Russian use of nuclear weapons and be prepared to f...