What started the war between the USSR & Finland?

On November 30, 1939, Soviet aircraft struck Helsinki, while the Red Army crossed the state border on the Karelian Isthmus and attacked Finnish troops. Thus, the Soviet-Finnish War, better known as the ‘Winter War’, began.

The armed conflict was preceded by lengthy negotiations, which had been intermittently ongoing since 1938. The Soviet side feared that, in the event of a war between the USSR and Germany, the Nazis would attack through Finnish territory. Moscow offered Helsinki an agreement to establish military bases on Finnish territory, but the Finns refused, citing their neutral status.

The negotiations culminated in October-November 1939. The USSR declared that the state border, 30 km from Leningrad, threatened the city's security. It demanded that Finland cede part of the Karelian Isthmus and several islands in the Gulf of Finland, as well as lease the Hanko Peninsula for 30 years for the construction of a naval base and the stationing of troops there.

In exchange, Finland could receive twice as much territory in Karelia. However, the Finns were unwilling to cede the strategically important region with the Mannerheim Line. Despite attempts by both sides to find a compromise, the negotiations failed.

The ‘Winter War’ proved a severe test for the Red Army, but ended with a Soviet victory. According to the Moscow Treaty of March 12, 1940, the Soviet Union acquired much more territory than it had previously demanded and also received the right to lease the Hanko Peninsula.

"War was necessary, because peace negotiations with Finland had yielded no results and Leningrad's security had to be secured," Stalin declared in April 1940. "There, in the West, the three greatest powers were at each other's throats. When would the Leningrad issue be resolved, if not under such conditions…"