The fragile nuclear order

Mostly for deterrence. A government that has nuclear weapons can threaten devastating retaliation, which makes rivals think twice before attacking in the first place. A smaller reason is prestige, since joining the nuclear club signals major-power status. Once one regional rival builds a bomb, its neighbor often feels forced to match it.

Yes, more than once. In 1983, Soviet officer Stanislav Petrov's early-warning system reported an incoming American strike, and he judged it a malfunction rather than pass the alert up the chain. His call is widely credited with stopping a retaliatory launch that would have escalated into full nuclear war.

It would mean a nuclear weapon is actually used, not just threatened. Most strategists expect that to trigger immediate retaliation rather than restraint, since backing down after a strike would look far riskier than responding. The exchange could escalate fast between nuclear-armed rivals, causing mass casualties well beyond the first strike.

Nine: the United States, Russia, the United Kingdom, France, China, India, Pakistan, Israel, and North Korea. That count has stayed the same for years, even though several of these countries keep expanding their arsenals, which is one reason experts increasingly question how stable this balance really is.