This is a study of the practice of the Roman Catholic priesthood and a history of French missionaries in the United States. From 1789 to 1865---from the beginning of the French Revolution to the end of the American Civil War---hundreds of Catholic priests and seminarians migrated from France to the United States and assisted in the establishment of new dioceses and church parishes stretching west from Maryland to Kentucky, and south from Missouri to Louisiana, Texas, and Alabama. They thought of themselves as missionaries in a "New World" composed of "heretical" Protestants and "indifferent" Catholics. In the course of their evangelistic endeavor, however, missionaries realized just how difficult it was to practice the priesthood in accordance with what they learned in French seminaries and what they knew Rome expected of them. They recognized just how uncomfortable it felt to serve as transnational arbiters of Catholic beliefs and practices between French, Roman, and American interests. This collective feeling of operating in-between ideal standards of the priesthood and actual circumstances of foreign missions convinced many missionaries of their vocational inadequacies and pastoral deficiencies. It also precipitated changes in the direction of the Catholic Church in the United States from a strictly Tridentine model of devotion and clerical authority to a transnational process dependent upon the everyday negotiations of priests and laypeople. The decision of French missionaries to justify the institution of slavery and support the Confederate cause of war, in particular, represented the reorientation of missionary Catholicism away from strictly European sources of authority and toward regional and national trends in American culture and politics.