Finnish – Estonian defence cooperation Finnish – Estonian defence cooperation began in 1930 with a secret military pact between Finland and Estonia against the threat of the Soviet Union . Open cooperation ended in 1939 , as the Soviets pressured the Estonian government , but it continued as secret information cooperation during the Winter War . Background . The origins of defence cooperation lay in the Baltic Entente of the 1920s . Practical cooperation began with the initiative of the Finnish general staff in February 1930 in Tallinn , the first joint war games were held in 1933 . Officially both countries were neutral , but behind the scenes the relationship was more complex . Nazi Germany , for example , unsuccessfully offered Estonia a secret military alliance in 1937 . Finland started cooperation for its own defensive reasons , not because both peoples speak Finnic languages . Defence plans involving Finland launching a massive assault against Leningrad if the Soviet Union started a war against both countries . However , the plan would also have required the participation of Latvia , and according to the Finnish military leader , Carl Gustaf Emil Mannerheim , the countries would have needed outside help from the League of Nations . Finland could not send troops directly to the Baltic countries , but a joint assault would have been possible through the Karelian Isthmus and / or from Poland . Gulf of Finland blockade plan . Before the 1930s , defence cooperation between Finland , Poland , Estonia , Lithuania , and Latvia was based on the Edge State Policy . In the 1930s this cooperation decreased , when Finland and Estonia started practical military dialogue . Due to the Finnish policy of neutrality and the official trend in the Nordic countries , the two nations kept their cooperation pact secret . The Nordic trend did not officially allow Swedish participation in managing the security of the Gulf of Finland . However , behind the scenes , the general staffs of Sweden and Finland had secretly negotiated a plan in 1929 for blockading the Gulf . Sweden agreed that it would first suggest the blockade plan to Estonia in 1930 . Officially , Sweden would not participate in the cooperation , but it would supply materiel and auxiliary troops if the Soviet Union attacked . Coastal artillery . Practical measures by Finland and Estonia were based upon Peter the Great ' s Naval Fortress system , allowing surface ships and coastal artillery fire to prevent possible movements of the Soviet Baltic Fleet . After the Imperial Russian Baltic fleet was mostly destroyed in the Russo - Japanese War of 1905 , Russia sought to replace the loss of fleet vessels from 1912 onwards by reinforcing the coastal artillery on the shores of the Gulf of Finland . The aim was to build a barrage against the German fleet ' s potential intrusion along the Gulf to Saint Petersburg , then capital of Russia . Coastal arrays of 12 - inch naval guns were installed on both sides of the Gulf . The defence cooperation aimed to prevent access by Soviet vessels through the Gulf to Helsinki and Tallinn using mine s , coastal artillery fire and submarine s . As a result , Finland redeveloped its heavy coastal artillery and fortresses , adapting 305 mm shells to give them greater range , thus providing complete artillery coverage between Mäkiluoto in Finland and Naissaar in Estonia . The first joint military exercise was held in 1936 . Submarines . The joint deployment of submarines was a part of the Gulf blockade plan . The Estonian submarine programme was expensive for the country , the Estonian Navy even had to sell two destroyer s in 1933 to be able to finance the two new submarines it launched in 1937 . Estonian submarines installed the same kind of torpedoes and mines as their Finnish counterparts , and Estonian naval officers were trained in Finnish submarines . Radio intelligence and the Winter War . During the Winter War , Estonia was formally neutral , even though it had to allow the Soviet Union to establish military bases and station troops on its soil . From those military bases , the Soviet Union carried out bombing raids on Finland . As part of the coastal artillery cooperation , the countries had a common fire management system linked by an undersea radio cable . The Estonians had earlier managed to decipher the Soviet secret radio code , and they had access to military radio communication s . During the Winter War , Estonian military staff gave the Finns top secret information on Soviet troop movements . Aftermath . The Finnish – Estonian defence cooperation pact remained secret for decades , and only came to light when the Estonian and Russian archives were opened in the 1990s . Finnish archives had been either transported outside the country or destroyed after the Continuation War .