Alliances are uncertain. A state may or may not come to the aid of an ally in a military crisis. Who is a threat to who is not always clear. States without reliable allies must find new means of security against a threat. Nuclear weapons heighten these threats. Security from nuclear attack concerns the continuity of society, not just military defeat. How does a state who once had—but now lacks—a dependable nuclear-armed ally secure themselves against an adversary’s nuclear threats? The most rational option is for these abandoned states to build nuclear weapons of their own. Proliferation models, threat response literature, theories of credibility, and the psychology of power stand at the center of abandoned state strategic rationality. I undertake a qualitative case study of French nuclear proliferation from 1954 through 1960 to uncover the exogenous, internal, and perceived factors informing abandoned state behavior.