What caused the winter war against Russia and Finland?

The question and answer needs qualification as to dates. Here's why;

Stalin's deal with Hitler via the Soviet -German Non Aggression act had secret clauses. Some of which of course divided up Poland between Germany & the Soviet Union. Also, the pact gave Stalin a free hand to invade Estonia, Bessarabia, Latvia and Finland. There may have been a misunderstanding between the two powers over the invasion of Finland. Therefore dates of the Soviet invasion needs clarification. In any case, Finland became a victim of Stalin's actions.
Now, of course the Pact did not preclude the Soviets from invading Finland. To add to the confusion surrounding Germany, Finland & the the USSR was the 1944 armistice between Finland & the USSR by which the Finns asked Germany to evacuate Finland.
Clarification on the Finland situation between 1939 and 1944 is welcomed.
============================================================================The Soviets were afraid of possible invasion by the Germans, that could take place through Finland or in the Gulf of Finland, so the USSR demanded the finns to move the border back 20km and rent an navalbase for the Soviets in the gulf of Finland.
After hard consideration, the finnish government said no, because it was clear, that the Soviets were going to invade rest of Finland afterwards. Stalin got angry and the Soviet generals made plans to invade Finland and make it look like Finland started the war.
The Russians ended up firing rounds from granadelaunchers next to border on their own side and they blamed the finns for it. The finns denied any firing because they didn't do anything and Stalin got an great opportunity to strike Finland.
Heavy bombardement started and attacks were launched across the whole border by the Soviets and so on...