Frossard briefly attempted to establish an independent Communist political organization before returning to the ranks of the SFIO, gaining election to parliament under that party's banner in 1928, 1932, and 1936.

Following the armistice between France and Nazi Germany, Frossard declined to participate in the Vichy French government headed by Pétain, but continued to work as a journalist.

[2] As the bloody conflict ground on without remit, Frossard's antiwar perspective became the majority view in the SFIO, leading to his election as Secretary-General of the party in 1918.

[2] In the summer of 1920 Frossard travelled to the Russian Soviet Federative Socialist Republic along with his party comrade, Marcel Cachin; the two participated in the 2nd World Congress of the Communist International.

[2] He traveled again to Moscow in June 1922 to serve as a delegate to the 2nd Enlarged Plenum of the Executive Committee of the Communist International (ECCI), a journey that marked his second and final trip to Soviet Russia.

[2] Although he did not attend the 4th World Congress of the Comintern in November 1922, he was still elected a member of ECCI at that gathering, his last high position in the French Communist movement.

[2] He initially attempted to form a dissident Communist group but ultimately failed in this task and returned to participation in the SFIO, now headed by Léon Blum.