In 1788, John Marshall made a prediction that was more prescient than he realized: The federal courts the new Constitution called for would be “the means of preventing disputes with foreign nations.” Marshall could not have known it, but for the next several decades international disputes over persons, ships, and goods caught up in maritime war would wash onto American shores, and into federal courtrooms. The courts’ decisions were essential to the United States’ emergence as a sovereign and independent nation. But preoccupation with Marshall’s famous constitutional rulings has obscured this story of judicial nation-building at sea. And while we have grown accustomed to the idea that “foreign affairs” are the domain of the legislative and executive branches, the political leaders who first tried to solve the puzzle of constitutional governance did not hew to such rigid notions of institutional responsibility. If Marshall’s legacy is the establishment of both judicial and national authority, this book shows that he and his contemporaries did so, first and foremost, at sea.