This book argues first, that the forces of industrialization that transformed ship technology simultaneously transformed the working-class lives of merchant seamen, intensifying class conflict and producing collective networks of subversion and resistance within the urban borderland spaces of sailortowns in which sailors fought to maintain control over their mobility, agency, and rights. Second, that given their social, cultural, economic, geographic, and legal marginalization, merchant seamen have occupied essential roles at the parameters of US urban, legal, labor, immigration, and wartime history. Third, that the constellation of these histories, embedded in the encounters and negotiations that merchant seamen provoked along the nation’s coastlines and sailortowns, collectively represents a unique and essential perspective on the history of US citizenship.