The nation-state, as it emerged in Europe during the nineteenth century, was perhaps the most paradoxical political institution of its age. Liberals endorsed nation-states, believing they would lead to peace, prosperity, and good government. But all too often they did the opposite. Reading Rousseau’s Social Contract against the eighteenth-century state system reveals one way in which political thinking at the end of the Enlightenment anticipated this paradox. Neither nationalism nor the nation-state were fully developed concepts at the time, though the glimpses of them that appeared in Rousseau’s works suggest just how problematic the emerging nation-state might be. Rousseau set out in the Social Contract to delineate the perfect republic. But the matter was not that simple. Rousseau did not confine his republic to an idealized setting. He placed it within an international order, and knew that it would need to defend itself. He may have designed the republic for liberty and self-government, but he equipped it for war. In the process, he endorsed a model of human association that, while suitable for defense, insisted on uniformity and would use coercion in order to achieve it. Rousseau’s Social Contract was a work of philosophy discussing ideas, not lived experience. But to the extent that it articulated a republican discourse that future nationalists such as Johann Gottlieb Fichte found useful, it provides insights into why the emerging nation-state was often accompanied by war, an emphasis on social uniformity, and a tendency toward authoritarian politics.