The 'Hanko Base Defemsive Operation' was the Soviet attempt to hold the Hanko base area in south-western Finland against the Finns (22 June/2 December 1941).

The operation took the form of a long series of small engagements on the Hanko peninsula during the 'Jatkosota' continuation war between Finland and the USSR. Each side was anxious to avoid any major, and therefore costly, battle and the fighting was therefore of the trench warfare type characterised largely by artillery exchanges, sniping, patrol clashes, and small amphibious operations in the surrounding archipelago.

The Moscow Peace Treaty, which formally ended the Soviet-Finnish 'Talvisota' winter war (1939/40) in March 1940, gave a lease of the Hanko peninsula to the USSR for use as a naval base on the northern side of the entrance to the Gulf of Finland, and the Finnish civil population was evacuated from the area before the arrival of the occupying Soviet forces. The leased area included several islands in the waters off the peninsula, a number of coastal artillery positions including the important fortress of Russarö, significant harbour facilities, and a flat area on which the Soviets quickly built an airfield. Troop transport rights between the USSR and Hanko severely strained Finnish/Soviet relations, and this was significant in Finland’s decision to allow German troops to transit northern Finland, and also in Finland’s later decision to go to war with the USSR in the 'Jatkosota'.

It is worth noting that while Hanko had originally been leased as a naval base, the Soviet ground forces in the peninsula were considerably more numerous than the naval forces, which comprised only a small detachment at the base.

Right from the start of the 'Jatkosota' on 25 June 1941, Finnish troops quickly isolated Hanko and its 25,300-man Soviet garrison, which rose to a maximum of some 30,000 during the siege. Though Sotamarsalkka Carl Gustaf Emil Mannerheim, the Finnish commander-in-chief, initially declared that the liberation of Hanko was a primary objective of the 'Jatkosota', the Finnish forces containing the Soviet lease area were not authorised to attack the base, but instead confined themselves merely to the occupation of the Harparskog line on the border of the leased area, built since the end of the 'Talvisota'. The front therefore remained effectively static, with operations taking the form largely of artillery bombardments and limited patrol activities by each side. Small-scale naval and amphibious actions also took place in the surrounding archipelago.

The Finnish forces surrounding the base initially comprised the 22,000 men of Eversti Aarne Snellman’s 17^th Divisioona (division), the 4^th Rannikon Prikaati (coastal brigade) and supporting units. By the end of the summer the 17^th Divisioona, which was the most important element of the besieging force, had been relocated to East Karelia, and the Finnish strength then stabilised at about 12,500 men. Finnish efforts to blockade the base from the sea were largely ineffectual as a result of strong Soviet resistance and Finnish equipment failures, the latter typified by the tendency of the torpedoes used by Finnish submarines not to detonate on impact. Minefields laid in the approaches 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 south shore of the Gulf of Finland, meant that the base soon lost its strategic significance and rendered it an unprofitably heavy burden for Vitse Admiral Vladimir F. Tributs’s Baltic Fleet.

So in the autumn of 1941 the Soviets decided to evacuate Hanko, and by December the base personnel, troops and most of the lighter equipment and supply stocks had been removed. Heavier equipment, which could not be readily removed, was sabotaged or destroyed. The Soviet naval forces responsible for the evacuation suffered heavy losses to mines.

The Soviet combination of the naval base at Hanko, the coastal fortress at Osmussaar and several minefields had hindered Finnish and German naval activities, and had made it problematic for freighters to reach the Finnish ports of Helsinki and Kotka. Finland lacked the resources to transport enough goods over land for ports on the eastern side of the Gulf of Bothnia farther to the north, and this caused severe logistic problems: matériel of many kinds therefore remained stuck in the ports of the west coast. Finnish and German minesweepers did manage to clear a lane through the minefields outside of the range of Russarö's artillery, thereby opening the chance for freighters to reach eastern ports, but it was not until the Soviet evacuation that they were able to clear the more secure coastal passage allowing safer passage.

The Finnish and Soviet coastal forces each undertook many small-scale amphibious operations in the archipelago surrounding the Hanko peninsula from a time early in July 1941, with operations ended in the following October. Fighting on these small islands was often fierce, but in general the operations had little effect on the overall battle as the territorial gains remained negligible. Typical of these amphibious operations was that on Bengtskär, a skerry held by the Finns. After capturing the small island of Morgonlandet in July 1941 the Soviets launched a small-scale amphibious assault on Bengtskär, which had Scandinavia’s tallest lighthouse and was thus an important observation post. The initial landing, performed in the middle of the night in foggy conditions, was successful, as the Finnish sentries believed the approaching boats to be German minesweepers, but the small garrison recovered quickly from its surprise. 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, and during the morning Finnish reinforcements were able to force the remaining Soviet raiders to surrender and drive their naval support away. The Finns lost 31 men killed and 45 wounded, while the Soviets suffered between 80 and 100 casualties and lost 28 men taken prisoner.

The Soviets undertook their evacuation of Hanko in several convoys between 16 October and 2 December, extracting some 23,000 men for movement to the east through the Gulf of Finland to Leningrad. In the course of the evacuation the Soviet naval losses to Finnish minefields and coastal artillery were three destroyers, the large transports Andrei Zhdanov and Iosif Stalin, and several smaller vessels. The Soviets also heavily mined the sea and land areas around Hanko, which slowed the Finnish reoccupation of the area.

While the overall Soviet losses are not known, those of the Finns were 297 men killed, 604 wounded and 78 missing.