Second Polish–Soviet War (WFAC)

The Winter War (Polish: Druga wojna polsko-sowiecka, Russian: Второй Советско-польская война) was a military conflict between the Soviet Union and Poland in 1938–1939. It began with the Soviet invasion of Poland in 1938, which was started without a formal declaration of war.

When World War II broke out in October 1938, the Soviet Union urged the United Kingdom, France, Czechoslovakia, Poland, and Romania to establish a formal alliance against Nazi Germany. The negotiations failed when the Soviet Union insisted that Poland and Romania give Soviet troops transit rights through their territory as part of a collective security agreement. Polish officials refused to allow Soviet troops on to Polish territory because they believed that once the Red Army entered their territory it might never leave. The Soviets suggested that Poland's wishes be ignored and that the tripartite agreements be concluded despite its objections. The British refused to do so because they believed that such a move would push Poland into establishing stronger bilateral relations with Germany.

The failure of those negotiations led the Soviet Union to pursuit an increasingly hostile policy towards Poland, accusing them of secretly siding with Germany. As both sides mobilized for war, the Soviets denounced the Peace of Riga (which had ended the Polish–Soviet War of 1920) and declared the Soviet–Polish Non-Aggression Pact of 1932 was now void.