In recent litigation before U.S. federal courts, the government has argued that military commissions have jurisdiction to prosecute offenses against the “common law of war,” which the government defines as a body of domestic offenses, such as inchoate conspiracy, that violate the American law of war. This Article challenges that definition by arguing that stray references to the term “common law of war” in historical materials meant something completely different. By examining the Lieber Code, the writings of early natural law theorists, and early American judicial decisions, this Article concludes that the “common law of war” referred to a branch of the law of nations that applied during internal armed conflicts, such as civil wars with non-state actors. This body of law was called “common,” not because it was extended or elaborated by the common law method of judge-applied law, but rather because it was “common” to all mankind by virtue of natural law, and thus even applied to internal actors, such as rebel forces, who were not otherwise bound by international law as formal states were. By recapturing this lost definition of the common law of war, this Article casts some doubt on the U.S. government’s position that military commissions have jurisdiction not only over international offenses, but also domestic violations of the law of war.