 On 26 November 1939 , a border incident was reported near the village of Mainila . A Soviet border guard post had been shelled by an unknown party resulting , according to Soviet reports , in the deaths of four and injuries of nine border guards . Research conducted by several Finnish and Russian historians later concluded that the shelling was carried out from the Soviet side of the border by an NKVD unit with the purpose of providing the Soviet Union with a casus belli and a pretext to withdraw from the non @-@ aggression pact . 
