Dark clouds loom on the nuclear horizon, with threats from all directions: Russia’s nuclear bombast in its war on Ukraine, China’s construction of hundreds of nuclear missile silos, North Korea’s missile testing, India and Pakistan’s ongoing nuclear competition, and Iran’s push toward nuclear weapons capability. In response, US policy-makers are discussing whether a further American nuclear arms buildup is needed. At the same time, evolving technologies, from hypersonic missiles to artificial intelligence, are straining military balances and may be making them more unstable. The risk of nuclear war has not been so high since the Cuban Missile Crisis.