On Nov. 30, 1939, Soviet troops crossed the border into Finland, beginning an unusual war that pitted small Finland against huge Russia.
I think something often overlooked about the Winter War was the overall context in which it occurred. It's easy to forget now, but 1939 was a year of extraordinary turmoil. The Spanish Civil War ended in January. Japan seized Hainan Island in February, Germany occupied the rump of Czechoslovakia in March, Italy occupied Albania in April and Germany invaded Poland on Sept. 1, starting World War II -- although in the fall of 1939 it was a Phony War confined to Europe and the high seas and not yet a world war.
So Finland's War with Russia was waged in a very tense international environment. It was co-existent with World War II, but not really a part of it. And even the later "Continuation War" waged in conjunction with the German Barbarossa campaign, was carefully separated from it. Finland was a co-belligerent, but not an ally, of Germany. The United States never did declare war on Finland, and Finnish restraint paid off when the tide turned against the Axis. Finland was the only Axis ally bordering the Soviets Union with was not occupied after the war.
I don't have a large collection of games on the Winter War, but one of my long-term favorites is the Strategy & Tactics game by that name, which I've even played this year. One of the TCS games, A Frozen Hell, depicts a battle from the war and several of my scenario-based wargames include episodes from the war, including Axis & Allies miniatures, Down in Flames and Memoir '44. I also have the GURPS WWII: Frozen Hell module from Steve Jackson Games which is full of interesting data about the war, even though I'm not an active player of thr GURPS RPG.
One aspect of the Winter War that's notable is the moral ambiguity surrounding it. On the one hand, the Finns were supported by Nazi Germany as well as the Western Allies, yet on the other they were opposed by the Soviets, who would be allied with the West against the Germans less than two years later. But the Finns were defending themselves against aggression, so Western and American sympathy was with them and their cooperation with the Germans was excused.