The Shelling of Mainila
Seven shots were fired, and their fall was detected by three Finnish observation posts.
These witnesses estimated that the shells detonated approximately 800 meters inside Soviet territory. Finland proposed a neutral investigation of the incident, but the Soviet Union refused and broke diplomatic relations with Finland on November 29.
Materials in the private archives of Soviet party leader Andrei Zhdanov heavily hint that the entire incident was orchestrated in order to paint Finland as an aggressor and launch an offensive. Some Russian historians have expressed doubts of the document's authenticity. The Finnish side denied responsibility for the attacks and identified Soviet artillery as their source — indeed, the war diaries of nearby Finnish artillery batteries show that Mainila was out of range of all of them, as they had been withdrawn previously to prevent such incidents.
In the days following the shelling, the Soviet propaganda machine generated much noise about other fictitious acts of Finnish aggression. The Soviet Union then renounced the non-aggression pact with Finland, and on November 30, 1939 launched the first offensives of the Winter War.
Faced with the Finnish intractability the Soviet Union finally decided to take by force what it could not get through negotiations. On November 30 1939 Russian troops entered Finland in a multitude of locations. Russia, and the world, expected a quick overrun-type operation. In a couple of weeks everything should be over.