[1] During the German Revolution of 1918–1919, Koenen was the commissar of the Worker and Soldiers' Council of the Halle-Merseburg district.

On 16 July 1919, in the National Assembly, he called for the adoption of a constitutional provision that would exclusively grant to the authorities and charitable organizations the right to hold public film screenings for adolescents so that youth would be protected from the wheeling and dealing of "the capitalists".

[3] In 1919, Koenen was a member of the board of the USPD's central committee, but in 1920, he joined the Communist Party (Kommunistische Partei Deutschlands, or KPD), where he became part of the left wing.

[5] Koenen left Germany in June 1933 on the decision of the party leadership,[1] first going to the Saarland, then still under foreign occupation.

He then went to France, where he was involved in the "Lutetia circle", trying to build a popular front against the Hitler régime.

Until 1946, he also worked as the chief editor of the KPD newspaper, Freiheit in Halle, forerunner of the Mitteldeutsche Zeitung.

[6] Koenen's son Heinrich was arrested by the Gestapo at the home of Ilse Stöbe on 29 October 1942, and in February 1945 was shot at Sachsenhausen concentration camp.