Why did Finland got involved in world war 2?

Finland became involved in World War Two on the side of the Axis powers when invaded by the Soviet Union on November 30, 1939 as a result of the same treaty in which Poland, Latvia, Lithuania and Estonia were divided between the Soviets and the Third Reich and Bessarabia was given to the Soviets. The Soviet Union was expelled form the League of Nations for this invasion, but the real consequences of the war were not felt until June 1941, 14 months after Finland's part in the war ended.

The Red Army invaded, but, despite the Soviets huge advantage in manpower and equipment, the Finns were able to defeat them for three reasons:

1) The Red Army depended on tanks and the Finnish terrain was not suited to tanks. Too many lakes and ponds, which when frozen made good tank traps.

2) The hand held machine gun called a "grease gun" (Suomi-konepistooli KP-31). The Finns were able to use it to make offensive warfare and infantry support of tanks difficult.

3) The use of bottles filled with gasoline to burn tanks. If you could keep the crews inside the tank and destroy the infantry support there are many blind spots that can be exploited to throw one of these on a tank. These were called "Molotov Cocktails" as an insult to the Soviet Foreign Minister and Politburo member, Vyacheslav Molotov.

The Finn's involvement ended in March 1940, when they ceded 9% of their territory, but the Red Army's terrible performance (Stalin had purged, which means murdered 50% of all officers just before war started, so the military leadership was inexperienced) led to Operation Barbarossa, the June 1941 Nazi invasion of the Soviet Union, which in turn led to Stalin "rehabilitating" (return from the Gulag) Zhukov, who then led the victory at Stalingrad which was the turning point for the war in Europe.