Even after decades of reducing their arsenals, the United States and Russia still possess more than 90 percent of the world’s nuclear weapons—over 8,000 warheads, enough for each to destroy the other, and the world, several times over. For a long time, both sides worked hard to manage the threat these arsenals presented. In recent years, however, geopolitical tension has undermined “strategic stability”—the processes, mechanisms, and agreements that facilitate the peacetime management of strategic relationships and the avoidance of nuclear conflict, combined with the deployment of military forces in ways that minimize any incentive for nuclear first use. Arms control has withered, and communication channels have closed, while outdated Cold War nuclear postures have persisted alongside new threats in cyberspace and dangerous advances in military technology (soon to include hypersonic weaponry, which will travel at more than five times the speed of sound).