Originally a member of the Democratic Union, Fabian became a member of the Social Democratic Party in 1912.
In 1914, when war broke out, he was arrested by an Italian ship in Marseille as an enemy alien with military service while on a Mediterranean voyage. Until 1918 he was interned as a civilian prisoner in Carcassonne and Ajaccio and was then exchanged for illness in Switzerland. There he joined the anti-war movement. In 1919 he returned to Berlin, where he became a member of Spartacists, the USPD and the KPD. In 1926 he was expelled from the KPD. Later he joined the KPD-O. On November 23, 1931 he resigned as a founding member of the SAP at. From 1925 to 1933 he was secretary of the Association of Socialist Doctors and editor of the journal The Socialist Doctor.
After the Nazi seizure of power, Fabian was listed as an enemy of the state and arrested. He was set free with a bail from his sister and moved to Prague. With the Germany invasion of Czechoslovakia, Fabian fled to Paris, and eventually New York, where he lived until his death in 1944.[1]