This Essay uses two 1804 opinions by Chief Justice John Marshall to explicate a world in which understandings of executive power and the rule of law were very different from those that predominate today. Scholars have misread Little v. Barreme and Murray v. Schooner Charming Betsy, this Essay argues, because they apply modern assumptions about the balance of power between Congress and the executive that do not fit the Marshall Court’s constitutional vision. Contemporary interpretations read Little for the propositions that the president’s inherent wartime power may be limited by statute and that early American jurists rejected officers’ “good faith” defenses to liability for tortious acts. But the opinion in fact reflects the Marshall Court’s view that, in an undeclared war, the president could not act at all unless authorized by Congress and that under no circumstances could the president give an officer a right to act where Congress had not. Charming Betsy, meanwhile, is known today for the “Charming Betsy canon”: Marshall’s assertion that wherever possible, courts ought to interpret the laws of Congress to accord with international law. In its historical context, however, the case illustrates Marshall’s view of the law of nations not as an external constraint on sovereignty—a common understanding of international law’s role today—but as an aspect of the rule of law critical to preserving the proper allocation of powers between Congress and the president. Indeed, read together, these cases show Marshall using the law of nations to reinforce a tenet central to the separation of powers in the new republic: that only Congress could alter the nation’s war footing. Through Little and Charming Betsy, the Marshall Court sought to foreclose Congress’s efforts to abdicate its responsibility to authorize acts of war and thus to underscore the constitutional balance that placed the warmaking and lawmaking power not with the courts, not with the executive, but with Congress.