Less than two years before the Soviet Union faced off against Nazi Germany during World War II, it waged a bloody war with another adversary: the tiny nation of Finland. Russia’s feud with its Nordic neighbor began in 1939, when Soviet leader Joseph Stalin looked to expand his influence over Eastern Europe. Citing concerns about a potential attack by the Germans, Stalin demanded that Finland’s border with Russia be moved back 16 miles along the Karelian Isthmus to create a buffer zone around the city of Leningrad. He also wanted the Finns to hand over several islands in the Gulf of Finland and lease the Soviets territory on the Hanko Peninsula for construction of a naval base. The Soviets offered a large swath of Russian territory as part of the deal, but the Finns were suspicious of their motives and turned them down. On November 30, 1939, following a series of ultimatums and failed negotiations, the Soviet Red Army launched an invasion of Finland with half a million troops.

Though vastly outnumbered and outgunned in what became known as the “Winter War,” the Finns had the advantage of fighting on home turf. Led by Marshal Carl Gustaf Mannerheim, they hunkered down behind a network of trenches, concrete bunkers and field fortifications on the Karelian Isthmus and beat back repeated Soviet tank assaults. Elsewhere on the frontier, Finnish ski troops used the rugged landscape to conduct hit-and-run attacks on isolated Soviet units. Their guerilla tactics were only aided by the freezing Finnish winter, which bogged the Soviets down and made their soldiers easy to spot against snowy terrain. One Finnish sniper, a farmer named Simo Häyhä, was eventually credited with over 500 kills.

While the Finns put up a spirited resistance during the winter of 1939-1940, their troops were ultimately no match for the sheer immensity of the Red Army. In February 1940, following one of the largest artillery bombardments since World War I, the Soviets renewed their onslaught and overran the Finnish defenses on the Karelian Isthmus. With its forces low on ammunition and nearing the brink of exhaustion, Finland agreed to peace terms the following month.

The treaty ending the Winter War forced Finland to cede 11 percent of its territory to the Soviet Union, yet the country maintained its independence and later squared off against Russia a second time during World War II. For the Soviets, meanwhile, victory came at a heavy cost. During just three months of fighting, their forces suffered over 300,000 casualties compared to around 65,000 for the Finns. The Winter War may have also carried important consequences for World War II. Among other things, the Red Army’s lackluster performance is often cited as a key factor in Adolf Hitler’s mistaken belief that his June 1941 invasion of the Soviet Union would be a success.