Claim: Eliezer Gruenbaum joined the Communist Party of Poland at the age of 16, then fought in the Spanish Civil War on the Republican side.
Evidence: At age of 16, he joined the Communist Party of Poland, and in 1929 was imprisoned by Polish authorities for four and half years for his communism-related activism (the Communist Party was at that time delegalized in Poland). He fought in the Spanish Civil War on the Republican side, and after returning to France, was once again arrested, this time by the French authorities, on charges of sedition (here, the accounts vary again, according to another source, he was arrested in Spain, but escaped safely back to France).