In December 1920 Dahlem took part in the "unification party conference" at which the left wing of the USPD merged with the German Communist Party to form what was briefly known as the ). Here, together with Philipp Fries and Henriette Ackermann, he was elected to the local party leadership of the Middle Rhine region. He also briefly represented Middle Rhine nationally on the party central committee. In 1921 he surrendered his editorship of the "Sozialistische Republik", but during 1921/22 he briefly edited the Berlin-based "Internationalen Presse-Korrespondenz". When the French army occupied the occupied the Ruhr region in January 1923 Dahlem helped to organise "resistance to French and German imperialism". He was sent by the German Communist leadership to Paris in order to coordinate with the French Communists against the common enemy. From 1923 Dahlem was working in the Organisation Department of the Party Central Committee. He was particularly effective in the application of "Leninist principles" to party organisation. In 1927 he himself joined the Central Committee, becoming a member of its Politburo just two years later.