Battle of Hanko ( 1941 ) The Battle of Hanko ( also known as the Hanko Front or the Siege of Hanko ) was a lengthy series of small battles fought on Hanko Peninsula during the Continuation War between Finland and the Soviet Union in the second half of 1941 . As both sides were eager to avoid a major , costly ground battle , fighting took the form of trench warfare , with artillery exchanges , sniping , patrol clashes , and small amphibious operations performed in the surrounding archipelago . A volunteer Swedish battalion served with Finnish forces in the siege . The last Soviet troops left the peninsula in December 1941 . Background . As part of the 1940 Moscow Peace Treaty which formally ended the Soviet - Finnish Winter War , Hanko was leased to the Soviet Union as a naval base . The civilian population was forced to evacuate before Soviet forces arrived . The leased area included several surrounding islands , several coastal artillery sites ( among them the important fort of Russarö ) , important harbor facilities , and an area suitable for an airfield , which the Soviets quickly constructed . Troop transport rights from the Soviet Union to Hanko and back put severe strain on Finnish - Soviet relations , and played a part both in Finland ' s decision to allow German troops to transit Northern Finland , and later , to go to war with the Soviet Union . Though Hanko had originally been leased as a naval base , ground forces were far more numerous , with only a small naval detachment being present at the base . Operations . At the start of the war , Finnish ground troops quickly isolated Hanko and its 25 , 300 - man Soviet garrison . Though Mannerheim initially declared that liberating Hanko would be a primary goal of the war , Finnish troops in the area did not receive authorization to attack the base . Instead , as the Finns had built the Harparskog line on the border of the leased area during the Interim Peace , they moved to occupy these positions . The front remained mostly static , with action consisting mainly of artillery strikes and some limited probing or patrol activities on both sides . Small scale naval and amphibious actions took place in the surrounding archipelago . Finnish forces surrounding the base initially consisted of the 17th Division , the 4th Coastal Brigade , and supporting units . By the end of the summer , the 17th Division , which had made up the bulk of the besieging force , was transferred to East Karelia . Finnish efforts to blockade the base from the sea were less successful , due both to strong Soviet resistance , and to equipment failures ( such as torpedoes used by Finnish submarines , which often failed to detonate on impact ) . Minefields laid on the sea lanes leading to Hanko and the surrounding waters were more effective , claiming several Soviet supply vessels . These problems , in addition to the rapid German advance on the southern shore of the Gulf of Finland , caused the base to lose its importance and made it an untenably heavy burden for the Soviet Baltic Fleet . In autumn 1941 the order was given to evacuate Hanko . Base personnel , troops and most of their light equipment and supplies had been removed by December 1941 . Heavier equipment which couldn ' t be readily moved was sabotaged or destroyed in place . Soviet naval forces performing the evacuation suffered heavy losses from minefields . The Soviet base at Hanko , its accompanying coastal fort at Osmussaar , and the minefields laid to protect the Soviet Baltic Fleet had hindered Finnish and German naval activities , and had made it problematic for freighters to reach the Finnish ports of Helsinki and Kotka . As Finland lacked the resources to transport enough goods over land this caused severe logistical problems , with material stuck in seaports on the Western coast . Finnish and German minesweepers had opened a sea - lane through the minefields outside of the gun range of Russarö to allow freighters to reach even the Eastern ports , but it wasn ' t until the Soviet evacuation that they were able to clear the more secure coastal sea - lane , allowing safer passage . Amphibious operations . Both Finnish and Soviet coastal forces conducted numerous small - scale amphibious operations in the archipelago surrounding the Hanko Peninsula . The first of the these clashes took place at the beginning of July 1941 ; active operations ended the following October . Fighting on these small islands was often fierce , and withdrawing from them under fire was extremely hazardous . In general the operations had little effect on the overall battle , as territorial gains remained negligible . Battle of Bengtskär . After capturing the small island of Morgonlandet in July 1941 , Soviet forces launched a small - scale amphibious assault against the Finnish island of Bengtskär , which had a lighthouse and was thus an important observation post . The initial landing , performed in the middle of the night in foggy conditions , was successful , as Finnish sentries believed the approaching boats to be German minesweepers ; however , the small garrison recovered quickly . Putting up fierce resistance , the Finns managed to retain control of the lighthouse while summoning help from nearby naval forces and coastal artillery . The fighting continued throughout the night . In the morning Finnish reinforcements were able to force the remaining Soviet raiders to surrender and drive their naval support away . Evacuation . The evacuation of Hanko was performed in several convoys , between October 16 and December 2 , 1941 , which managed to transport roughly 23 , 000 troops to Leningrad . The fleet suffered casualties from Finnish minefields and coastal artillery , losing 3 destroyers and 2 large transports ( " Andrei Zhdanov " and " Iosif Stalin " ) as well as several smaller vessels . Finnish troops entering the area found it heavily mined .