The Winter War (1939-1940)

The Molotov-Ribbentrop Pact between Nazi Germany and the Soviet Union included a secret protocol dividing Eastern Europe into spheres of influence, placing Finland, the Baltic states, and eastern Poland in the Soviet zone. The pact removed Germany as a check on Soviet policy toward Helsinki and set the stage for the territorial demands that followed.

Location: Moscow, USSR

Citing the security of Leningrad, roughly 32 kilometers (20 miles) from the Finnish border, Stalin demanded border adjustments on the Karelian Isthmus, leases on the Hanko peninsula, and the cession of islands in the Gulf of Finland — offering twice the land area of forested Lapland in exchange. Finland, having watched the Baltic states accept similar demands and lose independence, refused the core terms, and negotiations collapsed in November.

Location: Moscow, USSR

The Soviet Union claimed Finnish artillery had shelled the border village of Mainila, killing four soldiers — a staged incident later confirmed by Russian historians. The USSR broke its non-aggression pact with Finland and used the incident as justification for war two days later, without issuing a formal declaration.

Location: Mainila, Soviet Union

Soviet forces crossed the Finnish border on a broad front from the Arctic to the Gulf of Finland, expecting a quick victory against roughly 300,000 defenders. The Soviet Union simultaneously installed a puppet "Finnish Democratic Republic" under Otto Kuusinen at Terijoki, intending to rule Finland through it — a miscalculation that hardened Finnish resistance.

Location: Karelian Isthmus, Finland

In the frozen forests of central Finland, Finnish ski troops using motti encirclement tactics destroyed two Soviet divisions at Suomussalmi between December and early January, seizing their tanks and artillery. The victory at Suomussalmi — alongside Tolvajärvi — halted the Soviet advance north of Lake Ladoga and made the "Molotov cocktail" a symbol of improvised resistance.

Location: Suomussalmi, Finland

The League of Nations expelled the Soviet Union for its aggression against Finland — one of the only times the organization exercised this power. The expulsion carried little practical consequence, but underscored Finland's international standing and helped rally foreign sympathy and over 8,000 foreign volunteers, while actual military aid from Western powers remained limited.

Location: Geneva, Switzerland

After six weeks of failed offensives, Marshal Semyon Timoshenko took command of the Northwestern Front, massed overwhelming artillery, and retrained forces for a methodical attritional assault on the Karelian Isthmus. Soviet casualty estimates for the war range from roughly 130,000 to 170,000 dead — several times Finland's roughly 26,000 — a toll attributed to the purged officer corps, terrain, and winter.

Location: Leningrad, USSR

After a devastating barrage, Soviet forces breached Finnish fortifications at Summa, forcing a general retreat from the Mannerheim Line on February 15. Finland's appeals for Allied intervention came too late; with defenses collapsing and no foreign army arriving, Helsinki opened negotiations in late February while fighting a fighting withdrawal.

Location: Summa, Karelian Isthmus

The treaty ended the war on harsher terms than the original 1939 demands: Finland ceded about 10 percent of its territory, including the Karelian Isthmus and the shores of Lake Ladoga, and leased Hanko. Around 420,000 evacuated Karelians refused to remain under Soviet rule and were resettled across Finland. The war's outcome fed Finnish revanchism, leading to the Continuation War alongside Germany in 1941, and later to decades of armed neutrality.

Location: Moscow, USSR