In this American Academy of Arts and Sciences study, Rebecca Davis Gibbons, Stephen Herzog, Wilfred Wan, and Doreen Horschig unpack the challenges to the global nuclear order posed by the Russian war against Ukraine. On February 24, 2022, Russia invaded nonnuclear-armed Ukraine and leveraged threats with its nuclear arsenal as a “shield” to deter third-party intervention. The well-publicized horrors on the ground in Ukraine are, unfortunately, not the only consequences of Russia’s full-scale invasion of its neighbor. The war is having unmistakable effects on how governments, scholars, and the public think about nuclear arms. Not only has Moscow reintroduced the world to the often-unsavory realities of nuclear deterrence, but its suspension of the New Strategic Arms Reduction Treaty (START) and deratification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) have been setbacks for arms control and disarmament. Meanwhile, vulnerable states around the globe may be further incentivized to develop nuclear weapons or seek protection from nuclear-armed patrons to avoid being invaded like Ukraine. Given these changing geopolitical circumstances, how might the Russian war on Ukraine affect the global nuclear order? The authors in this publication conclude that the United States and the broader international community must now more seriously engage with alternatives to traditional arms control, nonproliferation, and disarmament endeavors. Specifically, the authors discuss the increasing prominence of approaches such as the Treaty on the Prohibition of Nuclear Weapons (TPNW)—popularly known as the Nuclear Ban—and risk reduction measures. They assess whether these initiatives can have an impact in reducing nuclear dangers. Additionally, they examine temptations for states to pursue more forceful counterproliferation measures and describe the risks of doing so.