In 1918 he was sentenced to 1 year imprisonment for his participation in an anti-war demonstration at Elbing throughout the German Revolution of 1918–19. After his release from prison Reimann moved to Ahlen in 1920 to work as a miner, joined the German Coalminer Union and became a full-time official of the Communist Party of Germany (KPD) in 1921. Reimann fought against the French Occupation of the Ruhr in 1923 and was imprisoned for a short time. Throughout the 1920s, he held several positions within the Revolutionäre Gewerkschafts Opposition (RGO), the Communist union in the Ruhr area.