In the complex web of history around World War II there are many intriguing story lines. There are threads with unintended consequences. In 1939 the Soviet Union invaded Finland. This invasion helped convince Hitler to invade the Soviet Union in 1941.
Why did the Soviet invasion of Finland help convince Hitler? Well... the Soviets had a very hard time invading Finland. The Finns essentially fought them to a draw and then negotiated a settlement. If a small band of Finns could hold off the Soviet Union, and in fact could humiliate it, then it must be easier to conquer then was once thought. The logic seemed flawless.
The logic was wrong of course. How one nation invades another is not necessarily a reflection of how it will defend itself on its home soil. How well the Soviets would fight to defend their homeland was different than how well they fought to invade Finland.
The logic was also wrong because it assumed the defeat of the Soviets was mostly due to the Soviets themselves. The tenacious ability of the Finns wasn't considered as important.
And were the Finns ever tenacious, tactically sound, and able. A Thousand Lakes of Red Blood on White Snow by Arto Bendiken is a wonderful explanation of how able the Finns were. It's an amazing story and you're guaranteed to learn some details you never knew before. Do you know why volatile liquid in glass bottles with a lit wick is called a Molotov Cocktail? Arto will tell explain. All while showing how a small nation of determined people humiliated one of the world's superpowers to defend their country and their way of life.