By restricting the permissible justifications for war and excluding all tangible benefits from the permissible goals of war, replacing both with an amorphous, however seemingly narrow interest in “defense,” those clear yardsticks have been lost. As a result, we no longer have a clear metric of success that marks the sufficiency of the force used: there is no recognizable moment in which the war has achieved its legitimate goals. Victory can no longer be measured by concrete benefits but only by the absence of concrete harms. And an absence is hard to prove. Modern wars, as a consequence, may have more morally legitimate reasons, but they are also more difficult to judge and to restrict.
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The justifications and goals of war may have always been indeterminate, at least to some extent. But unlike before, as international lawyers, we are confined today to the question whether these justifications and goals contribute to defense in a necessary and proportionate manner. That question, I argue, is unanswerable.