Dethmann was active in the Communist Party of Germany (KPD), editing the journal Spartakus - the KPD journal for Schleswig-Holstein from March 1919. However he was thrown out of the party following the Heidelberg conference in October 1919, and became a founder member of the Communist Workers Party of Germany (KAPD) when that was founded in April 1920.
- Bourrinet, Philippe (2016). The Dutch and German Communist Left (1900–68): ‘Neither Lenin nor Trotsky nor Stalin!’ - ‘All Workers Must Think for Themselves!’. BRILL. ISBN 9789004325937. Retrieved 27 September 2018.
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