[1] He attended school in Jena and after the end of the First World War joined the Free Socialist Youth, the forerunner of the Young Communist League of Germany.

After passing his apprenticeship exam, Poser worked as a travelling journeyman carpenter which took him through Switzerland, Austria, Hungary, Czechoslovakia, Denmark, Finland and the Soviet Union.

On 26 September 1936, a few weeks after his release, he married Lydia Orban, whom he had known for a long time from the Young Communist League and who had also been sentenced to two years in prison with him.

Poser's organization had connections to Franz Jacob and Anton Saefkow in Berlin as well as the anti-Nazi military opposition around Claus von Stauffenberg and the Kreisau Circle.

After undergoing interrogation and torture, he allegedly tried to flee on the night of 20 July 1944, but was said to have been hit by five shots in an adjacent park.