This Article seeks to develop a clear and sensible legal standard governing defensive first strikes writ large in inter-state conflicts. Imprecise or improperly gauged legal parameters can contribute to an increased risk of hostilities, whether due to abuse, error, or even reasoned calculation. The implications of such conduct for states and their populations alike can be enormous. Although many proposals posit constructive guideposts for such a standard, they tend to be either abstract in structure or limited in material application. This Article sets forth a legal standard that aims to be simultaneously systematic in approach, comprehensive in scope, and functional in operation—all while embracing the elemental virtues of clarity and realism. After defining the presumptive baseline standard, the Article sets forth a legal policy framework upon which to erect a standard premised on several key attributes and contemporary security circumstances while fostering legal legitimacy and diminishing the incidence of armed conflict. The specific proposal for reform consists of three substantive elements: an evidentiary standard, a set of procedural safeguards, and a standard of review. The Article also compares the prevailing standard with the proposed standard and evaluates the latter’s prospects for adoption.