The Winter War (Finnish: talvisota, Swedish: vinterkriget, Russian: Зимняя война, tr. Zimnyaya voyna) was a military conflict between the Soviet Union and Finland. It began with a Soviet offensive on 30 November 1939—two months after the start of World War II and the Soviet invasion of Poland—and ended on 13 March 1940 with the Moscow Peace Treaty. The League of Nations deemed the attack illegal and expelled the Soviet Union from the League on 14 December 1939.

The Soviet forces had more than three times as many soldiers as the Finns, thirty times as many aircraft, and a hundred times as many tanks. The Red Army, however, had been crippled by Soviet leader Joseph Stalin's Great Purge of 1937, reducing the army's morale and efficiency shortly before the outbreak of the fighting. With more than 30,000 of its army officers executed or imprisoned, including most of those of the highest ranks, the Red Army in 1939 had many inexperienced senior officers. Because of these factors, and high commitment and morale in the Finnish forces, Finland was able to resist the Soviet invasion for far longer than the Soviets expected.

Simo Häyhä (Finnish pronunciation: [ˈsimɔ ˈhæy̯hæ]; December 17, 1905 – April 1, 2002), nicknamed "White Death" (Russian: Белая смерть, Belaya Smert; Finnish: valkoinen kuolema; Swedish: den vita döden) by the Red Army, was a Finnish sniper. Using a modified Mosin–Nagant in the Winter War, he has the highest recorded number of confirmed sniper kills – 505 – in any major war.

Häyhä was born in the municipality of Rautjärvi near the present-day border of Finland and Russia, and started his military service in 1925. Before entering combat, Häyhä was a farmer and a hunter. At the age of 17, he joined the Finnish militia suojeluskunta and succeeded with his sniping skills in shooting sports in the Viipuri province. His farmhouse was reportedly full of trophies for marksmanship.

During the Winter War (1939–1940), between Finland and the Soviet Union, he began his duty as a sniper and fought for the Finnish Army against the Red Army. In temperatures between −40 and −20 degrees Celsius, dressed completely in white camouflage, Häyhä was credited with 505 confirmed kills of Soviet soldiers. A daily account of the kills at Kollaa was conducted for the Finnish snipers. Remarkably, all of Häyhä's kills were accomplished in fewer than 100 days – in other words, an average of 5 kills per day – at a time of year with very short hours of daylight.