For years, historians have argued that Catholicism in the United States stood decisively apart from papal politics in European society. The Church in America, historians insist, forged an “American Catholicism,” a national faith responsive to domestic concerns, disengaged from the disruptive ideological conflicts of the Old World. Drawing on previously unexamined documents from Italian state collections and newly opened Vatican archives, Peter D’Agostino paints a starkly different portrait. In his narrative, Catholicism in the United States emerges as a powerful outpost within an international church that struggled for three generations to vindicate the temporal claims of the papacy within European society.Even as they assimilated into American society, Catholics of all ethnicities participated in a vital, international culture of myths, rituals, and symbols that glorified papal Rome and demonized its liberal, Protestant, and Jewish opponents. From the 1848 attack on the Papal States that culminated in the creation of the Kingdom of Italy to the Lateran Treaties in 1929 between Fascist Italy and the Vatican that established Vatican City, American Catholics consistently rose up to support their Holy Father. At every turn American liberals, Protestants, and Jews resisted Catholics, whose support for the papacy revealed social boundaries that separated them from their American neighbors.