Altiero Spinelli was the main editor of the Ventotene Manifesto (1941), a kind of trusty mirror of what the Federalist Resistance thought and projected, a manifesto which long inspired and mobilized a part of the young people who militated outside the traditional political circles. His very conversion to Europe reveals a new kind of commitment that changed its nature in the face of war and moved from theory to militant practice. Spinelli, by his itinerary, was exemplary of thousands of conversions due to new hopes and a new context resulting from the disaster. His manifesto therefore fits perfectly into the post-war history of federalism, a failure report, and at the same time the expectation of a renewal. With this eminently political text, federalism leaves the scholarly books of law or the talented but isolated essays of the 1920s, to change into a hopeful message and a reconstruction model for the future.