From the publisher: This article evaluates whether the length of time states have nuclear weapons influences their behavior and the behavior of opponents in militarized disputes. Using multiple statistical models and illustrative cases, the article shows that, while acquiring nuclear weapons makes states significantly more likely to reciprocate militarized challenges and have their challenges reciprocated, over time, the effect reverses. In contrast to a static understanding of nuclear weapons, this variation in outcomes over time highlights the difficulties presented by nuclear proliferation.