Complex sentence: During the German Revolution of 1918 he became chairman of the newly formed Workers and Soldiers council of Cologne, which successively exercised authority over the fortress of Cologne, which had tens of thousands of retreating, demoralized soldiers.
Split into simpler sentences: During the German Revolution of 1918 he became chairman of the newly formed Workers and Soldiers council of Cologne. This council then successfully exercised authority over the fortress of Cologne, which had tens of thousands of retreating, demoralized soldiers.