Description
Nuclear security guarantees are central to the global security order, yet the notion of the “nuclear umbrella” remains conceptually underspecified and inconsistently applied, often inferred from outcomes or equated with alliance membership. This paper (1) reviews the existing literature to identify where and how nuclear umbrellas are theorized to affect outcomes such as deterrence, nonproliferation, alliance cohesion, free-riding, and moral hazard; (2) derives a rigorous empirical definition based on three necessary criteria—a declared nuclear capability, an explicit commitment that includes nuclear use, and at least one ex ante costly signal enhancing credibility; (3) applies this framework to construct a global dataset of nuclear umbrella relationships since 1945; and (4) replicates two prominent empirical studies to provide theoretically grounded evidence of the effects of nuclear umbrellas on international security outcomes.