He participated in a strike of the Stone Mason Union against the beginning rearmament in Kiel in 1931 caused by the launching of the German pocket battleship Deutschland.
Since he had been actively publishing for the Kommitee für Recht und Freiheit (Committee for Law and Freedom) in Zürich, he took part from November 1936 on the side of the Second Spanish Republic in the Spanish Civil War as a member of the International Brigades.
After the resignation of the Republic in 1939, Maaßen would be jailed and spent the following year in prisons and camps of Francoist Spain.
Hanns Maaßen started worked as a commentator at Sender Leipzig where he would be dismissed in 1950 because of "leftist allowances".
Hanns Maaßen wrote essays on literary themes narrative works in which he processed the experiences of his many year Spanish stay.