Leading into World War II there were a number of smaller conflicts that fed into the larger inferno. One was the "Winter War" between the old Soviet Union and Finland. The Soviets in a pact with Germany who had just invaded Poland launched a full scale invasion of Finland.
Surprisingly the Finns almost held the massive Soviet Union to a stalemate. While the Russians recovered some territory it lost in World War I to Finland, that nation largely survived the brutal assault from its neighbor. Near a quarter of million Russian soldiers died in this 'minor war' while the Finn's lost only 30,000 of their military.
Many believe the Russians poor showing in the war against much smaller Finland gave confidence to Hitler to attack their former ally