There are books about the Winter War. Winter of 1939-1940. USSR invaded Finland to grab territory. So it was national defense. Finns fought on, severely outnumbered but expert in winter warfare, surrounding numerous Soviet columns and cutting them into battalion-size pockets, and doing raiding attacks on ski. (
Battle of Suomussalmi)
Stalin killed his best generals, by first accusing them of imaginary crimes, then saying they did commit those crimes (i.e. a communist purge), and had unjust execution of all of them. To kill off anyone competent enough to potentially lead the country, that way he had complete safety of power.
So the Soviet troops did terrible in the Winter War. If the Finns had enough reinforcements, they would have massacred the entire Soviet army in front of them. But there were not enough people of military age to replace the dead Finns, even if the kill ratio was like 6 to 1. The Soviets had tanks and air planes. The Finns didn't. So the Soviets got a piece of land on the Karelian peninsula, though considering their casualties it really wasn't much.
Finland is the country in the middle. Karelian peninsula is all that is red on the bottom.
That's the Winter War. Quite glorious. There are books about it (one titled "The Winter War"), really good read. There is also a movie about it "Talvisota". The cinematography is awesome. Please watch it.
When Hitler invaded the USSR in June 1941, the Finns went into war as well. They didn't go because they signed a pact with the Axis side, they went to war to recover the land that Stalin stole.
The Finns also have a military symbol that is a half-Swastika, but they had it as a Scandinavian symbol long before Hitler was born, and it has nothing to do with national socialism.
So they fought until they recovered their land lost in the Winter War. Then they stopped just outside of Leningrad, tying up many Soviet units, but not actively fighting/invading for the Germans.
Later the Germans lost the Battle of Kursk 1943 (Stalingrad 1942 was bad, but not as bad as Kursk) and the tide turned. So the Soviets invaded Finland again, and they fought the Soviets to a ceasefire.
movie: Tali-Ihantala I actually haven't read any book on the Continuation War, but it should be very interesting.
I know nothing about the Lapland War till today. I don't think the Finns really fought the Germans because the Germans never occupied Finland (the Finns were as good as, or better fighters than, the Germans) and as far as I know the only German units inside Finland were "lend-lease" Luftwaffe units. The Germans also sold to Finland (I don't think they gave for free) quality materiel like anti-tank weapons (Panzerfaust and Panzerschreck) and assault guns (Sturmgeschütz), which consist of a non-rotating turret built on the tracks of a tank. It is way harder to maneuver in combat since you have to rotate the entire vehicle, but it is cheaper and faster to manufacture. Without those, the Finns would have had a difficult time stopping the T-34s, who were much better built than the T-70 and T-26 of the Winter War. So I really don't think the Finns would fight the Germans when the latter helped them survive the hordes of Stalin.
