Controversy and confusion pervade the law of armed conflict. Its most basic rules may seem ambiguous, vague, incomplete, or inconsistent. The prevailing view of customary international law confronts serious problems, in principle and in practice, when applied to the customary law of armed conflict. This article examines different forms of legal indeterminacy and different legal techniques available to address them, using concrete controversies to illustrate abstract ideas. It defends one view of the purpose of the law of armed conflict and its relationship with other rules of international law. The purpose of the law is not to balance a constraining principle of humanity against an authorizing principle of military necessity. Instead, the purpose of the law is simply to protect persons and objects to the greatest extent practically possible, that is, without depriving other rules of international law, which authorize certain uses of armed force, of practical effect. Finally, it shows that the law of armed conflict contains a number of clues for its own interpretation, some of them hidden in plain sight, including a recurring pattern of general protections with limited exceptions.