On 30 November, Soviet forces invaded Finland with 27 divisions, totalling 630,000 men, bombed civilian boroughs of Helsinki and quickly reached the Mannerheim Line. The shelling of Mainila was a "casus belli" of the Soviet Union as it had withdrawn from non-aggression pacts on 28 November. Germany had staged a similar incident to have an excuse to start war against Poland. The Soviet Union would later use the Orzeł incident to challenge the neutrality of Estonia.