In 1939 Eugenio was arrested and sent to confinement on the island of Ventotene.
Among the other prisoners and friends of Eugenio Colorni on Ventotene were Ernesto Rossi and Altiero Spinelli, who in 1941 co-authored the famous Ventotene Manifesto "for a free and united Europe", i.e. an early sketch of a postwar democratic European Union.
The Ventotene Manifesto is a political statement written by Altiero Spinelli and by Ernesto Rossi while they were prisoners on the Italian island of Ventotene during World War II.