Preemptive War

The concept of preemptive war refers to a legitimate anticipatory war waged by a state in order to counter a threat emanating from an adversary. Like many other notions used as much by politicians as by scholars, preemptive war is an essentially contested concept, all the more so as it cannot easily be dissociated from the concept of preventive war.

The idea of a legitimate anticipatory war goes back to the founders of the modern just-war tradition in the 17th and 18th centuries. Writing about the lawfulness of wars, Hugo Grotius and Emer De Vattel claimed that natural law as well as the law of nations permitted states to repel force by force in order to defend themselves against danger. They acknowledged two types of danger: First, a state is in danger once it has been attacked, and in this case resorting to arms is legitimate for the sake of self-defense; second, a state is in danger when an attack against it is imminent, and in this case resorting to arms is legitimate to forestall the intended attack. The criteria put forward by Grotius and De Vattel introduced a breach in the then prevailing Hobbesian conception of international politics—considering any war to be justified per se and postulating that a danger, and an ensuing right to prevent such a danger, existed as soon as there was a real or a perceived increase of another state's power capacities. During the so-called Caroline incident opposing the United States [Page 2121]and the British empire at the Canadian border in 1837, the criteria put forward by Grotius and De Vattel were revisited by U.S. Secretary of State Daniel Webster; according to Webster, an anticipatory war could be considered to be legitimate on the condition that it was undertaken as a necessity of self-defense, instant, overwhelming, and leaving no choice of means and no time for deliberation.

From that time on, it has been generally acknowledged that a distinction must be made between legitimate anticipatory wars, called preemptive wars, and illegitimate wars, called preventive wars. Although both relate to the same better-now-than-later logic and designate an anticipatory and winnable war waged by a state eager to forestall a threat at time t in order not to have to wage a riskier war under less favorable circumstances at times t + 1, t + 2, …, t + n, the material dissimilarity of the threat concerned induces a normative difference between them.

Concerning the material dimension, the first difference lies in the temporality of the threat countered by an anticipatory war; a preemptive war is undertaken to confront a threat perceived to be imminent, whereas a preventive war aims at forestalling a threat likely to become effective only after a certain period of time. In other words, a preemptive action is a tactical riposte to a short-term threat, while a preventive action is a strategic reply to a long-range threat, such as a future imbalance of forces. The second material difference, complementary to the first one, relates to the source of the threat. In a preemptive war, State A faces an irresistible threat: It attacks at the point in time t when an adversary, State B, is about to mobilize actual military capacities against A, whereas a preventive war launched by State A seeks at time t to impede State B from building up military capacities that might be mobilized against A at times t + 1, t + 2, …, t + n. In other words, a preemptive war aims at countering a real threat, whereas a preventive war strives to forestall a hypothetical threat.

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