The Soviet Union had signed international and mutual nonaggression treaties with Finland: the Treaty of Tartu of 1920, the Non-aggression Pact between Finland and the Soviet Union signed in 1932 and again in 1934, and further the Charter of the League of Nations. The Soviet government attempted to adhere to a tradition of legalism, and a casus belli was required for war. Earlier in the same year, Nazi Germany had staged the similar Gleiwitz incident to generate an excuse to withdraw from its nonaggression pact with Poland. Also the Soviet war games held in March 1938 and 1939 had been based on a scenario where border incidents taking place exactly at the village of Mainila would have sparked the war.