Leading analysts have predicted for decades that nuclear weapons would help pacify international politics. The core notion is that countries protected by these fearsome weapons can stop competing so intensely with their adversaries: they can end their arms races, scale back their alliances, and stop jockeying for strategic territory. But rarely have theory and practice been so opposed. Why do international relations in the nuclear age remain so competitive? Indeed, why are today's major geopolitical rivalries intensifying? This text tackles the central puzzle of the nuclear age: the persistence of intense geopolitical competition in the shadow of nuclear weapons.