While prosecuting its invasion of Ukraine, Russia has relied heavily on nuclear threats, turning the war in Ukraine into a dangerous nuclear crisis with profound implications for the global nuclear order and its two constitutive systems of nuclear deterrence and nuclear restraint. These two interconnected systems, each aiming to manage nuclear possession and reduce the risk of nuclear use, are at once complimentary and contradictory. While tensions between these systems are not new, the war in Ukraine exacerbates them in unprecedented ways. The system of nuclear deterrence seems to be proving its worth by inducing restraint on Russia and NATO, while the system of restraint is undermined by demonstrating what happens to a country not protected by nuclear deterrence. The latter lesson is particularly vivid given Ukraine’s decision to forgo a nuclear option in 1994 in exchange for security assurances from nuclear powers. Russia’s use of nuclear threats as an enabler for escalation and the specter of Russian tactical nuclear use against Ukraine goes well beyond its declared nuclear doctrine. The outcome of the war in Ukraine thus has critical importance for deciding the value of nuclear weapons in global security architecture and for resolving the conundrum between the systems of deterrence and restraint.