In the period between the two world wars, the coastal defence batteries situated around Tallinn formed a system of naval fortresses – Estonia’s naval fortresses. Estonia’s naval fortresses were subunits of the Estonian naval forces and simultaneously also under the command of covering military units. The task of the naval fortresses was to defend Tallinn against threats coming from the sea in the event of a general mobilisation. According to the plan for covering the border, the coastal defence batteries of the naval fortresses had to prevent enemy warships from invading the Bay of Tallinn from the Suurupi-Naissaare-Aegna line and also to prevent the bombardment of the city from the sea. The other tasks of the naval fortresses included preventing the enemy from making amphibious landings within firing range of the coastal defence batteries, supporting the warships of the Estonian naval forces, and defending naval navigation and mine barrages laid in the sea.

By 1939, the naval fortresses had a developed command structure. Each fortress had its own commander as well as a general staff. In regions where coastal defence batteries were situated, three commandant’s offices were established: the commandant’s offices of the Suurupi, Naissaare, and Aegna naval fortresses.

Simultaneously with the occupation of Estonia on 17 June 1940, the Red Flag Baltic Fleet also seized Estonia’s naval fortresses. On 6 July 1940, an agreement was signed between the Republic of Estonia and the Soviet Union according to which the USSR formally rented the coastal defence batteries located at Aegna, Naissaare, Suurupi, and Viimsi for 10 years. After the agreement was signed, the coastal defence batteries and other infrastructure of the commandant’s offices of the naval fortresses started being handed over to the Soviet Union and civilians who lived in those regions were ordered to leave.

Statements were drawn up regarding the property, structures, and tracts of land that were to be handed over to the Soviet Union. Assessments of the monetary value of the coastal defence batteries were carried out. At the same time, in July of 1940, soldiers’ committees that dealt with spreading communist propaganda started being formed in Estonian military units, including at the commandant’s offices of the naval fortresses.

By mid-July of 1940, all of the coastal defence batteries of the commandant’s offices of the naval fortresses had been handed over to the Soviet Union. The liquidation of the commandant’s offices of the naval fortresses officially began on 3 August 1940 when Johannes Santpank, the commander of Estonia’s naval forces, issued a directive ordering the formation of commissions for liquidating the commandant’s offices of the naval fortresses. It should be noted that this was a part of the liquidation of the Estonian Army. Similar processes took place in Latvia and Lithuania as well.

The liquidation commissions of the commandant’s offices of the naval fortresses started handing over the archives of the commandant’s offices from the storehouse of the Naval Forces Base, along with parts of artillery guns, optical sights, etc. Some of the personnel of the naval fortresses were dismissed from active service and sent into reserve. Soldiers in active service were transferred to the ranks of the Naval Forces Base. At the same time, military personnel who had served in naval fortresses were also transferred to other Estonian military units.

A joint liquidation commission for the commandant’s offices of naval fortresses was formed on 9 September 1940. It started handing insignia of the naval fortresses over to the Estonian War Museum, and it transferred documents to the Riigi Keskarhiiv (National Central Archive, nowadays the Estonian National Archives). Similarly, simultaneously with the commandant’s offices, their officers’ clubs and non-commissioned officers’ clubs were liquidated. On 28 September 1940, the liquidation statements for all three commandant’s offices of the naval fortresses were submitted to the commander of the naval forces. With that, Estonia’s naval fortresses as a subunit of the naval forces were liquidated.