In The National Security Strategy of the United States of America, released one year after the 9/11 attacks, the Bush administration asserted that “we must adapt the concept of imminent threat to the capabilities and objectives of today’s adversaries.” The administration’s reconceptualization has triggered an intense debate within academic and policymaking circles about the legal and ethical distinction (if any) between anticipatory and precautionary self-defense, and thus preemptive and preventive warfare. We begin by examining the present status on international law on the question of anticipatory self-defense. We then consider how the formative texts on international law—especially the writings of Gentili, Suarez, Grotius, Pufendorf, and Vattel—might inform the current debate, and ask if contemporary legal distinctions can be sustained in light of the jus ad bellum principles put forward by early thinkers in the just war tradition. Our aim is to ascertain whether a review of the classic literature supports a revision of current international law and, if so, the extent to which such a revision might be compatible with the Bush administration’s doctrine of preemption.