The principle of proportionality, notoriously obscure in application and subjective in interpretation, has been enforced so rarely as to call into question its potency as a meaningful international legal standard. Nonetheless, international criminal tribunals, academics, and the ICRC's monumental study on customary international humanitarian law all confidently proclaim the principle as embedded in the customary international law applicable to both international and non-international armed conflicts. To assess whether these claims are accurate, and to flesh out how states interpret the principle in practice, the author and a colleague have undertaken a long-term, multinational empirical study of state practice in interpreting and enforcing the proportionality principle. This article discusses the methodological options available and explains the one chosen for the proportionality study. The limitations of the study, in spite of its deliberate methodology, suggest that the debilities of the proportionality principle may not be conceptual as much as a byproduct of unnecessary military secrecy. This article concludes that greater transparency in state compliance with the rule of discrimination and the principle of proportionality would, at least, facilitate an understanding of how the hitherto obscure principle operates in practice and, at best, could create systemic effects that would decrease the dangers to civilians in armed conflicts.
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