Humanity has lived for generations beneath weapons capable of inflicting unimaginable destruction. Their continued existence has often been defended as a means of preventing war. That argument becomes more dangerous when the states holding those weapons start behaving as if the old constraints no longer matter.

In its June 2026 yearbook, the Stockholm International Peace Research Institute reported that all nine nuclear-armed states continued modernizing their arsenals in 2025. Many deployed new nuclear-armed or nuclear-capable systems. SIPRI describes a broader pattern in which nuclear weapons are regaining importance as instruments of national power, even as risks of misunderstanding and escalation rise.

Modernization is not the same event as an imminent launch. It would be irresponsible to turn evidence of an arms race into a numeric probability of nuclear war without a valid model. Our forecast is deliberately narrower: the continuation of modernization programmes. Its high probability reflects documented programmes, not confidence that catastrophe will happen.

The long-term stakes, however, are staggering. Weapons increase the number of paths by which miscalculation could become irreversible. Treaties, inspection mechanisms and crisis communication can reduce those paths. The danger is not simply the technology. It is the decision by leaders to weaken the institutions that once limited its use.