The book analyses the Europeanist and federalist effort of Italian Protestants in the struggle for European unification. This investigation revolves around two distinct guiding arguments: a political one, focused on the analysis of political thought; and a historical one reconstructing the most recent events about the Italian Protestants' activism for the political unification of Europe. The essay retraces the developments of federalism within the Protestant world from the 16th to the 20th century by referring to the bond between federalism and ecumenism. The volume is divided into three parts and provides a historical overview of federal thought within the Protestant world from the Reformation to the Enlightenment. It also addresses a series of projects aimed at the political unification of the European continent, and analyses the similarities between ecclesiastical constitutionalism and institutional federalism. This theoretical background paves the way for the contribution of Italian Protestants to the international peace movement and the confessional reconciliation among the Churches in the 19th and 20th centuries. Finally, this essay highlights the practical and theoretical contribution of the Italian Protestants to the cause of United States of Europe, according to the principles of the Ventotene Manifesto.