 Jahn was arrested three times by the Nazis, the last being in 1935, after they discovered his resistance group.He fled to Prague, then on to Amsterdam, where from 1936 to 1938, he published a journal for German railway workers in opposition to the Nazis.In 1940, he escaped to London, travelling via Spain and Portugal, where he set up a group of German trade unionists which later affiliated to the National Group of German Labor Unionists in Britain, and also served on the council of the ITF.