We develop a formal model of bargaining between two states, where one can invest in a program to develop nuclear weapons and the other imperfectly observes its efforts and progress over time. In the absence of a nonproliferation deal, the observing state watches the former's program, waiting until proliferation seems imminent to attack. Chance elements-when the program will make progress and when the other state will discover this-determine outcomes. Surprise proliferation, crises over the suspected progress of a nuclear program, and possibly "mistaken" preventive wars arise endogenously from these chance elements. Consistent with the model's predictions and contrary to previous studies, the empirical evidence shows that the progress of a nuclear program and intelligence estimates of it explain the character and outcomes of most interactions between a proliferant and a potential preventive attacker. Counterintuitively, policies intended to reduce proliferation by delaying nuclear programs or improving monitoring capabilities may instead encourage it.

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We present a model of the relationship between the spread of new military technologies and the occurrence of war. A new technology could shift the balance of power, causing anticipatory war as one side tries to prevent the other from obtaining it. When one side already has it, war is more likely when the shift in power is large, likely, and durable. When neither side has it, war is more likely when the expected shift is asymmetric (e.g., one side is more likely to get it) and when the two sides fear that a war will occur once one of them has it. We illustrate the model with historical examples from the spread of firearms (the Musket Wars in pre-colonial New Zealand) and of nuclear weapons (the end of US nuclear monopoly, and the 1967 Six-Day War). A broader implication is that major power competition can unintentionally cause wars elsewhere. * Comments are welcome and should be sent to mbas@gov.harvard.edu and acoe@fas.harvard.edu. We are grateful to

Arming is puzzling for the same reason war is: it produces outcomes that could instead be realized through negotiation, without the costly diversion of resources arming entails. Despite this, arms control is exceedingly rare historically, so that arming is ubiquitous and its costs to humanity are large. We develop and test a theory that explains why arming is so common and its control so rare. The main impediment to arms control is the need for monitoring that renders a state’s arming transparent enough to assure its compliance but not so much as to threaten its security. We present evidence that this trade-off has undermined arms control in three diverse contexts: Iraq’s weapons programs after the Gulf War, great power competition in arms in the interwar period, and superpower military rivalry during the Cold War. These arms races account for almost 40% of all global arming in the past two centuries.