During the Finnish-Soviet negotiations ending the Continuation War (1941–1944) between Finland and the Soviet Union, the Soviets renounced their claim to Hanko and instead demanded the right to construct a naval base at Porkkala. The Finns were forced to agree to a fifty-year lease on Porkkala as part of the Moscow Armistice concluded on 19 September 1944. Finnish President Carl Mannerheim regarded this concession as the most troublesome provision of the armistice agreement because Soviet forces could easily occupy Helsinki via Porkkala.
The Finnish handover of Porkkala took place ten days after the armistice was signed, and its 10,000 inhabitants were given eight days to evacuate the peninsula. On 28 September 1944, Soviet troops occupied the area. The lease was confirmed in the Paris Peace Treaty of 10 February 1947. On 24 May 1947, the Soviet Union and Finland signed the Porkkala-Udd Transit Agreement that permitted Soviet forces to transit southeastern Finland in order to supply their recently acquired base.
In the years that followed, the value of the Porkkala Peninsula to the Soviet Union diminished. There was no threat to Soviet control over the southern coast of the Gulf of Finland, and military advances also meant that the Soviet Union could close off the Gulf of Finland from its southern coast. Also, conclusion of the Finnish-Soviet Treaty of Friendship, Cooperation, and Mutual Assistance in April 1948 and the policy of cooperation with the Soviet Union pursued by President Juho Paasikivi (1946–1956) all worked to lessen tensions between the two states.
On 17 February 1955, Soviet Prime Minister Nikolai Bulganin informed President Paasikivi and Prime Minister Urho Kekkonen that Moscow was willing to renounce its fifty-year lease on Porkkala in return for a twenty-year extension of the Treaty of Friendship, Cooperation, and Mutual Assistance with Finland, and a communiqué was subsequently issued to that effect. Porkkala was officially returned to Finland on 26 January 1956.
Vehvilainen, Olli. Finland in the Second World War: Between Germany and Russia. London: Palgrave, 2002.