At the close of the Winter War (1939-1940), the Moscow Peace Treaty (13 March 1940) leased Hankoniemi as a naval base to the Soviet Union for 30 years. Finland ceded the Hanko region to the representatives of the Red Fleet on Good Friday, 22 March 1940.
During the spring, summer and fall of 1940, the Soviet Union moved a total of 30,000 of its troops to Hankoniemi. It justified the base by wishing to close the mouth of the Gulf of Finland with mines and artillery fire. These weapons operated together with the Soviet artillery on Osmussaari and Hiidenmaa’s Tahkunanniemi in the south part of the Gulf of Finland.
The Soviet Union defined the aims of the Hanko naval base as follows:
— The closing of the Gulf of Finland with sea mines and long-range artillery together with the artillery on the Estonian islands in the south part of the gulf.
— The defence of the base against attacks from sea, land and air by utilizing the navy, air defence, air force and ground forces.
The Hanko region was entered by particularly many building troops – totaling around 10,000 – whose output is still visible in the numerous fortifications that populated Hankoniemi. Heavy minage supplemented the defence. The Finnish Defence Forces still has to clear this minage yearly on land and the Hanko archipelago.
— The focal point of the fortifying was the tip of the Hankoniemi and the archipelago that surrounds it.
— The stations fortified the most are located in Russarö.
— Also the peninsulas and islands that emerge from the Hanko city center are heavily fortified.
The heavy battery cannon positions in Tulliniemi have preserved well and constitute an appropriate monument of the Hanko Soviet base, in particular as plans abound to transform Uddskatan into a recreation area.
The air-defence of the fort of Hanko was strong. Hanko posed altogether four separate antiaircraft divisions (corresponds with a battalion), each of which had three four-cannon batteries. The equipment of the batteries was 76 millimeter Russian air defence cannons, whose Finnish military symbol is 76 ltK/31.