My dissertation examines the role of “the law of nations”—as international law was known in the eighteenth-century—in the diplomacy of the American Revolution. My method is to assess the way in which European and American diplomats used this law in a series of negotiations involving Spain, France, Britain, and the United States in a conflict over the Mississippi River. I argue that European statesmen based their conduct on a set of pragmatic norms, derived from precedent, which were known as the customary law of nations. American revolutionaries were generally naïve in their use of this law, having had no prior experience with international affairs. John Jay—the emissary tasked with defending American interests in the Mississippi—was an exception to this rule. Among Jay’s most notable attributes was the tenacity with which he defended the statehood of the United States, and its corresponding right to the privileges and protections afforded by the law of nations. The issue lay at the heart of the conflict over the Mississippi, and Jay’s conduct, I demonstrate, was decisive to its outcome. In my last two chapters I explore the source of Jay’s perspicacity and suggest that he likely derived his understanding of the law of nations from the treatises of Samuel von Pufendorf—a leading proponent of a theoretical version of the law of nations that was popular in intellectual circles at the time. Pufendorf was an authority in “moral philosophy”, or the scientific study of natural moral law; and he defined states as corporate moral persons, whose rights derived from a universal law of sociability. Jay was educated at King’s College in New York City (1760-1764), and the president of King’s, Samuel Johnson, was one of the preeminent authorities in British North America on Enlightenment-era theories of natural law. Johnson gave Pufendorf a central place in his curriculum, and it was Pufendorf’s theories, I argue, combined with the authority with which Johnson imbued them, that lay behind Jay’s use and conception of the law of nations.