The nuclear question has returned to the center of global politics.

While the specter of nuclear proliferation never disappeared, it was obscured for decades by a functioning and predictable global order, underpinned by a hegemonic United States, a strong NATO and credible arms-control regimes. But this order is now under unprecedented strain, with the U.S.-Israeli war on Iran just the latest evidence. How can we preserve nuclear restraint in a world in which the architecture of restraint is crumbling?

The dawn of the nuclear age led to an inversion of strategic thinking. Until then, military power had been measured by the capacity to win wars, which was tested on the battlefield. But nuclear weapons’ purpose was deterrence, not victory.