In 1936 he was arrested by the Swiss authorities; he escaped the threat of extradition to the German Reich by fleeing to Spain.
[2] As a member of the International Brigades, Claudius was one of the first to take part in the Spanish Civil War on the Republican side as a foreign volunteer.
Because he was staying in the country illegally, he was arrested again and had to spend the years from 1939 to 1945 as an internee in various Swiss labor camps.
The threat of deportation to the German Reich was prevented by interventions by the authors Hermann Hesse and Albert Ehrenstein.
[2] Claudius' literary work consists of novels, short stories, reports, memoirs, travelogues and dramas.