One of the many unintended consequences of the Russian invasion of Ukraine is the renewed discussion among politicians, as well as in the popular media, of nuclear war. President Putin may have cast the first stone, but many others have quickly followed. Would Russia have dared invade Ukraine if that latter had not traded in their stockpiles of nukes in the 90s? Would Saddam Hussein or Muammar Gaddafi still be in their palaces if they had nuclear weapons to scare off the Americans and the Europeans? Are the North Koreans simply internalizing those lessons, and why does anybody think that the Iranians have not drawn similar conclusions, especially since their arch enemy Israel crossed the nuclear threshold a long time ago?

How worried should we be about Russian or Korean or Chinese or prospectively Iranian nukes? Have we suddenly moved from a world of nonproliferation to a world of proliferation?

Dr. Tytti Erästö is an expert on nuclear weapons at the Stockholm International Peace Research Institute (SIPRI). For her, a key assumption has long been that rational leaders would never use nuclear weapons. But now she is asking, what about irrational ones?