On 26th November, 1939 at 15 hours 45 minutes, Soviet troops located near the village Mainila, at the border with Finland were unexpectedly fired by [Finnish] artillery fire. A total of seven shots were fired. As a result, three soldiers and one junior commander were killed. Seven soldiers and two commanders were injured. The Soviet government informed the Finnish side with a note. Finland answered with their own note and stated that the fire originated in the Soviet territory – Finnish boarder guards did not possess the weapons to do that. Moscow was not satisfied and called its diplomats from Finland.
In the morning November 30 the Red Army crossed the Finnish border. The war started.
It is an undisputed historical fact that the shelling of Mainila was a false flag operation by the Soviet Union. It gave the USSR a pretext to launch the Winter War.
Later, Russia has admitted setting up the incident.