States invoke economic crises and security threats to justify treaty non-compliance. The most dramatic recent examples of this phenomenon include "necessity" defences in international investment law; "emergency" derogations in international human rights treaties; "exceptions" for non-conforming measures in international trade law; and doctrinal misapplications of necessity in jus ad bellum and jus in bello. Necessity and National Emergency Clauses is the first to trace the doctrine's genealogy from medieval Christian and Islamic religious history to post-Westphalian practices, the...