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This paper is a response to Curtis A. Bradley & Mitu Gulati, Withdrawing from International Custom, 120 Yale LJ 202 (2010), which argues against the "Mandatory View" (according to which states are bound by customary international law with no possibility of opting out), and in favor of a "Default View" which permits states to opt out of international custom unilaterally. My response offers the following arguments: (1) Currently, the most significant contested issue about customary international law in U.S. discourse concerns the laws of war -- a topic that Bradley and Gulati treat only briefly and incidentally. Their proposal would make it possible for the United States to withdraw unilaterally from customary law-of-war limitations. (2) Part of Bradley and Gulati's case for the Default View is that it actually represents the historical norm until the twentieth century. I argue that their sources don't adequately support this claim. Their main source, Vattel, thought that states can opt out only of a customary rule that is "indifferent in itself"-- a category that excludes many important rules of customary international law, including the jus in bello rules of the law of war. I discuss other sources as well. (3) Bradley and Gulati believe that the Mandatory View was a colonialist invention to lock new nations into old rules, but I argue that the history they cite does not support this diagnosis. (4) Turning from history to policy, permitting states to opt out of the law of war would likely have immediate dangerous effects on the ground as the U.S. military rewrites its manuals and retrains officers and troops to respond to changes in law. The result of a US opt-out is more likely to be an unraveling of the law of war than a helpful revision leading to better rules.

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120 Yale L.J. (Online) 151-167 (2010)