Why did the Soviet Union invade Finland starting the Winter War in 1939?

The USSR invaded Finland because, in those days, the border between the two countries was different than today. An artillery gun, quite some distance from the border, could drop shells into the city centre of Leningrad (called St Petersburg today). Stalin, paranoid as he was, wasn't so keen on that so he had an incident concocted, the shelling of the Soviet village of Mainila, to justify the USSR withdrawing from their Non-Aggression Treaty with Finland.

Shortly thereafter the Soviets invaded, after ensuring that the Germans would not interfere, and the war was on. It's worth noting that the Finns outfought the Soviets by a huge, unheard-of degree. 70,000 casualties for the Finns and 323,000 casualties for the Soviets. That's almost a five to one ratio.

Stalin got around this disparity by sending huge numbers of soldiers - being 5 times better than your enemy doesn't mean much if your enemy sends 10 times as many soldiers - and the Finns were pushed back and had to accept peace on Soviet terms.

Once World War Two started, the Finns allied themselves with the Germans so they could retake their territory but it's worth noting that they didn't do much beyond that - they just wanted their land back. At the end of the war, to avoid the USSR taking their whole country, they surrendered the retaken territory again.

In the end however, it has to be said that the Finns lost the war. They have never regained their lost territory and they aren't likely to get it back in the future.

The USSR invaded Finland because Finland would not become a puppet on the USSR and would not surrender territory to the USSR. Look at a map. Finland borders the former USSR and it's border was VERY close the Leningrad (now St Petersburg) which is the 2nd most important city in Russia. The Soviets, of course, won - BUT, the Finns fought them very well and repulsed them for a very long time. What the Finns did by holding off the Soviets was nothing less than a miracle and the whole world saw it as such. They lost in the end, of course, butm what they accomplished was magnificent.

There are great books on the subject and I urge you to read at least one.

The USSR never invaded Finland in 1939, it was Nazi Germany. Hitler wanted world domination and started with Finland because Germany had motorized vehicles while Finland still had horses for calvary. This invasion started World War II.

UPDATE: I know more about WWII than you idiot.

to steal finland back from the nazis. russia wanted to grab as much real estate as it could before the US came in and made their sweep.