 In early September 1917, following the German conquest of the Russian port of Riga, the German navy decided to eliminate the Russian naval forces that still held the Gulf of Riga. The Admiralstab (the Navy High Command) planned an operation to seize the Baltic island of Ösel, and specifically the Russian gun batteries on the Sworbe Peninsula. On 18 September, the order was issued for a joint operation with the army to capture Ösel and Moon Islands; the primary naval component was to comprise the flagship, Moltke, along with the III and IV Battle Squadrons of the High Seas Fleet. Along with nine light cruisers, three torpedo boat flotillas, and dozens of mine warfare ships, the entire force numbered some 300 ships, supported by over 100 aircraft and six zeppelins. The invasion force amounted to approximately 24,600 officers and enlisted men. By this time, the IV Battle Squadron had come under the command of Vice Admiral Wilhelm Souchon. Opposing the Germans were the old Russian pre-dreadnoughts Slava and Tsesarevich, the armored cruisers Bayan, Admiral Makarov, and Diana, 26 destroyers, and several torpedo boats and gunboats. The garrison on Ösel numbered some 14,000 men.
