Operation Albion was the German land and naval operation in September-October 1917 to invade and occupy the Estonian islands of Saaremaa (Ösel), Hiiumaa (Dagö) and Muhu (Moon), then part of the Russian Republic. The land campaign opened with landings at Tagalaht Bay, Saaremaa, on 11 October 1917, after extensive naval activity to clear mines and subdue coastal artillery batteries, and the island was secured by 16 October. The Russian Army evacuated Muhu on 18 October.
After two failed attempts, the Germans managed to land on Hiiumaa on the 19th and captured the island on the following day. The Russian Baltic Fleet had to withdraw from Muhu Sound after major losses (see Battle of Moon Sound). The Germans claimed 20,000 prisoners and 100 guns captured during Operation Albion from 12 October.
At the beginning of World War I the islands were of little importance to either Imperial Russia or Germany. However after the revolutionary turmoil in Russia during 1917, the German high command believed capturing the islands would outflank Russian defences and lay St Petersburg vulnerable to attack.