This dissertation argues that the United States owed much of its early success, as well as certain aspects of its identity, to the marine insurance companies that began to form in American port cities after the ratification of the Constitution. When the American states chartered insurance companies as "corporations and bodies politic," they granted these companies new legal privileges. Insurance companies, however, were built around older bodies of organized capital and commercial information. Marine insurance was an age-old financial practice that ordered and disciplined merchant commerce. It functioned, in fact, as one of the key governance mechanisms of what we can understand as a polity of merchants, bound together more broadly by the set of customs, ideas, and technologies known as lex mercatoria, the law-merchant. This polity of merchants predated the United States by centuries; its relationship with governments was by turns antagonistic and complementary. As the early modern English state expanded its support for merchant commerce and the lucrative business of marine insurance, the polity of merchants increasingly came to be articulated through the state's new institutions. The American states chartered more than one hundred insurance companies between the ratification of the federal Constitution and the end of the Napoleonic Wars. These incorporations did not signify the full assimilation of commercial wealth into the republic. Rather, the new insurance companies became the leaders of a highly capitalized and coordinated American insurance sector that retained its capacity for independent action. In fact, insurers' legally secured status at home enabled them to shore up their control over American commerce overseas, establishing themselves anew as governors of the polity of merchants. At the same time, American insurance companies developed deep and mutually beneficial relationships with the other new American bodies politic, particularly the state-chartered banks and the federal government itself. The relationships among insurance companies, banks, and government, I argue, formed the core of a new American political economy. For insurers, securing a place in the United States was a cultural project as well as a legal and economic one. Through the deliberate efforts of company leaders, as well as through the productions of contemporary mapmakers, biographers, and fiction writers, the first generation of financial corporations came to be seen as a group of stable and patriotic institutions. This cultural entrenchment of the polity of merchants shaped public understandings not only of the American financial sector but also of the national project as a whole. The relationship between marine insurers and the federal government set the terms for American experiences in the French Revolutionary and Napoleonic Wars. Insurers provided significant capital and information resources to the government, though they did so largely on their own terms. Insurers, uniquely, were perceived to stand both inside and outside the republic: they were celebrated as bulwarks of American commerce, but also respected as independent and objective assessors of American commercial security. When the Napoleonic Wars ended, and the existential threat to American trade subsided, insurers' political influence diminished. However, the project of establishing a workable body of American commercial law was just beginning. The importance of this project extended beyond the handful of American port cities dominated by commerce and insurance: the intellectual framework of nineteenth-century American nationalism itself was developed by lawyers immersed in the logic of lex mercatoria. Rather than viewing insurance as a response to risk in general, or as a mechanism for the accumulation of capital, this dissertation understands marine insurance first and foremost as a form of governance. American marine insurers, in particular, held unique political authority during the French Revolutionary and Napoleonic Wars. Throughout this period, American insurers remained as mercantile as they were American; they made markets, and they made states.