Winter War
The 105-day war in which Finland held off a Soviet invasion in 1939, lost a tenth of its territory, and kept its independence.
On 30 November 1939 the Soviet Union invaded Finland with an army several times the size of anything the Finns could field, and expected to be done in a fortnight. Finland held for 105 days. When it ended in March 1940 the Finns had lost about a tenth of their country and kept their independence, and the Red Army had lost roughly 127,000 dead and missing against Finland's 26,000. Hitler watched the whole thing and concluded that the Soviet Union was militarily hollow, which is one of the reasons he invaded it eighteen months later.
The war came out of the Molotov-Ribbentrop Pact of August 1939, whose secret protocol placed Finland in the Soviet sphere. That autumn Moscow demanded that Finland move the border on the Karelian Isthmus back from Leningrad, which sat about 20 miles from Finnish territory, hand over islands in the Gulf of Finland, and lease the Hanko peninsula for a naval base, offering a larger but useless stretch of Karelian forest in exchange. Finland refused. On 26 November Soviet artillery shelled their own village of Mainila and blamed the Finns. Four days later the invasion began.
Why it went wrong for the Soviets
The Red Army had just been through Stalin's purge of its officer corps, and it showed. Soviet divisions advanced along the few forest roads in long columns with their vehicles nose to tail, in temperatures that reached forty below. Finnish ski troops in white cut the columns into segments, sealed both ends, and destroyed them piece by piece, a tactic the Finns called motti, after the word for a stack of cut firewood. At Suomussalmi and on the Raate road in January 1940 a Finnish division wiped out an entire Soviet division that badly outnumbered it.
The Finns were short of everything, above all anti-tank guns, and improvised with bottles of fuel that they named Molotov cocktails, a joke at the expense of the Soviet foreign minister who had announced that Soviet aircraft were dropping food to the people of Helsinki. The sniper Simo Hayha, working in white camouflage with iron sights, is credited with more than 500 kills in about three months.
None of it changed the arithmetic. In February the Soviets stopped trying to maneuver, massed artillery on the Karelian Isthmus, and ground through the Finnish line by weight of shell. Finland asked for terms.
The peace and its long shadow
The Moscow Peace Treaty of 13 March 1940 took about 35,000 square kilometers, roughly eleven percent of Finland, including the whole Karelian Isthmus and Viipuri, the country's second city, plus Salla, several islands, and a thirty-year lease on Hanko. Around 400,000 Karelians left their homes and were resettled in what remained of Finland. Almost none chose to stay behind and live under Soviet rule.
Finland fought again from 1941 to 1944, this time alongside Germany, in what Finns call the Continuation War, and lost again. The 1947 peace treaty confirmed the 1940 borders and added reparations. In 1948 Finland signed a treaty of friendship and mutual assistance with Moscow, promising to resist any attack on the Soviet Union through Finnish territory and to stay out of alliances Moscow considered hostile. That bargain, a free hand at home in exchange for a foreign policy approved in Moscow, is what the world came to call Finlandization.
The Winter War is also why Finland kept universal conscription, an artillery park out of all proportion to its size, and hundreds of thousands of trained reservists through the Cold War and long after it. When Russia invaded Ukraine in February 2022, Finnish public support for NATO membership went from under a third before the invasion to more than three-quarters by that spring. Finland joined on 4 April 2023, and NATO's land border with Russia more than doubled.