15 Winter 2026 Webster’s formulation, which left open the possibility of anticipatory self-defense in certain circumstances, became known as the “Caroline doctrine.” The doctrine has been defined as follows: [U]se of force by one nation against another is permissible as a self-defense action only if force is both necessary and proportionate. The first of these conditions, necessity, means that resort to force in response to an armed attack, or the imminent threat of an armed attack, is allowed only when an alternative means of redress is lacking. The second condition, proportionality, is linked closely to necessity in requiring that a use of force in self-defense must not exceed in manner or aim the necessity provoking it.23 Britain reasserted the Caroline doctrine many years later in the 1893 Fur Seal Arbitration. Britain argued anticipatory self-defense could be justified in emergency situations, “when there is no time for deliberation, no time for contrivance, no time for warning, no time for diplomatic expostulation.”24 One early commentator endorsed the legality of anticipatory self-defense, arguing States have the legal right to use force “by preventive means if in its conscientious judgment necessary, against attack by another State, threat of attack, or preparations or other conduct from which an intention to attack may reasonably be apprehended. In doing so it will be acting in a manner intrinsically defensive even though externally aggressive.”25 A later commentator explained: Even internal law recognizes that acts committed in self-defense to avert an illegitimate act which has “commenced” or is impending are “legitimate.” In this situation it would evidently be impossible to expect that the party attacked should wait and try to obtain his rights by asserting them before the courts. The same must be all the more valid in International Law where there is no organized administration of justice to turn to.26 Another commentator, in his treatise on self-defense and international law, urged a broad interpretation of the right of anticipatory self-defense: [T]he right [of self-defense] under traditional international law, has always been “anticipatory,” that is to say its exercise was valid against imminent as well as actual attacks or dangers.... No state can be expected to await an initial attack which, in the present state of armaments, may well destroy the state’s capacity for further resistance and so jeopardize its very existence.27 Another commentator noted the interpretational difficulty that would arise if Article 2(4) were construed to limit the right to self-defense in Article 51 solely to situations where an actual armed attack occurred first: [T]he question of who decides when an armed attack occurs ... a question determined by the Charter, loses any special significance, for the right of selfdefense is not confined to that eventuality. The question is in fact a broader one as to who decides, in the first place, whether the occasion has arisen which justifies the exercise of the right of self-defense. In the nature of things this must be a decision for the state invoking the right, but this does not imply that the states unilaterally determines the legality of the action as is taken, for this final decision ought properly to be made by an impartial body and not by the parties concerned.28 23. Id. at 498 (emphasis added); see also R. Jennings, “The Caroline and McLeod Cases,” 32 AM. J. INT’L L. 82 (1938). 24. Fur Seal Arbitration (U.S. v. Gr. Brit.), Trib. Arb. 299 (1893), available at https://archive.org/details/ fursealarbitratipt13beri/page/n5/mode/2up 25. John Westlake, INTERNATIONAL LAW, PART I: PEACE (Cambridge Univ. Press 1910); see also Elihu Root, “The Real Monroe Doctrine,” 8 AM. J. INT’L L. 427, 432 (1914). 26. Alf Ross, A TEXTBOOK OF INTERNATIONAL LAW (Longmans, Green & Co. 1947). 27. D. W. Bowett, SELF-DEFENSE IN INTERNATIONAL LAW 187-90, 191-92 (Manchester University Press 1958). 28. Id. at 193.
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