In Paris, where he had worked as correspondent of the Avanti in 1921, he became acquainted with Léon Blum (socialist Prime Minister of France from 1936 to 1937), Marcel Cachin, Romain Rolland and Georges Sorel.
After being liberated in August 1943, he returned to Rome to lead the Italian Socialist Party, which had been reunified as the Italian Socialist Party of Proletarian Unity.