International law and armed conflict exist in a symbiotic relationship. In some cases, law shapes conflict proactively by imposing normative limits in advance of the appearance of proscribed conduct. Much more commonly, armed conflict either reveals lacunae in the law or demonstrates how law designed for yesterday's wars falls short when applied to contemporary conflict. When that happens, international law reacts by allowing provisions to fall into desuetude, embracing new interpretations of existing prescriptions, or generating new norms through practice or codification. In the 21st...