Operation Kilpapurjehdus ("Regatta") was the covername for the militarization of the Åland islands. The operation took place on June 22, 1941, and in this way, Finland strived to prevent Soviet landings in the area.
The Åland islands had been demilitarized since 1921, through a League of Nations decree, which outlawed any military personnel or material on the islands. After the end of the Winter War, the Soviet Union controlled both the Hanko Peninsula and Estonia and the Finnish leadership feared that the Soviets were planning a landing in the demilitarized area. The Finnish militarization plan was ready by April and they started to move troops towards the coast between Turku and Pori according to the plan.
The Navy HQ gave its own order for execution of the operation june 20, during daytime on 21 June 1941 Finnish warships had already relocated to the area of Nauvo Korppoo, and were followed by transports with troops in the evening. Although during the night they stopped on the border of the territorial waters to await confirmation of the German attack on the USSR, and after it was received at 4:30 the movement continued. The operation was launched at the same time as the German Operation Barbarossa. The landing involved over 20 cargo ships, the two coastal defence ships Ilmarinen and Väinämöinen, as well as the three gunboats Uusimaa, Hämeenmaa and Karjala. More than 5,000 men and artillery were landed the same evening in Mariehamn. Soviet aircraft tried to attack the ships during transport, but without result.
The Soviet Union had also created a similar plan after the Winter War, but they never tried to land on the islands during World War
The Soviet invasion threat was no longer acute after the Soviets had evacuated their Hanko base and the Germans had advanced in the Baltic states. The Finns started to transport the troops towards the eastern front. The Åland islands were again demilitarized after the end of the Continuation War and remain so today.
Operation Kilpapurjehdus became violation of Treaty concerning the Åland Islands between Finland and the Union of Soviet Socialist Republics concluded on 10 November 1940