Do states need to possess nuclear weapons to coerce their rivals? This chapter argues that the threat to proliferate alone is sufficient. To test the theory, I gather data on nuclear proficiency and compare it to outcomes of militarized interstate disputes. When an average country with low nuclear proficiency initiates a militarized dispute, I estimate that its target reciprocates in kind roughly 68% of the time. However, a country with high proficiency sees its disputes reciprocated only about 42% of the time. The 26 percentage point decrease is both statistically and substantively significant, thereby providing solid evidence that defending states internalize their opponents' potential nuclear capabilities in times of crisis.