 During the First World War he joined the Italian armed forces and fought in the alpine campaign, rising to the rank of second lieutenant.After the war, thanks to his brother Nello, he studied in Florence with Gaetano Salvemini, who was to be from then a constant companion of both the Rosselli brothers.It was in this period that he became a socialist, sympathetic to the reformist ideas of Filippo Turati, in contrast to that revolutionary thinking of Giacinto Menotti Serrati.