It was during the Winter War (1939–1940), between Finland and the Soviet Union, that 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 whitecamouflage, Häyhä was credited with 505 confirmed kills of Soviet soldiers. A daily account of the kills at Kollaa was conducted for the Finnish snipers. Besides his sniper kills, Häyhä was also credited with over 200 kills with a Suomi KP/-31submachine gun. Remarkably, all of Häyhä's kills were accomplished in fewer than 100 days at a time of year with very short hours of daylight. The unofficial Finnish front line figure from the battlefield of Kollaa places the number of Häyhä's total kills at over 800.
Häyhä used a Finnish militia variant, White Guard M/28 "Pystykorva" or "Spitz", of the Russian Mosin-Nagant rifle, because it suited his small frame (5 ft 3 in/1.60 m). He preferred to use iron sights rather than telescopic sights to present a smaller target (the sniper must raise his head higher when using a telescopic sight), for more reliable visibility (a telescopic sight's glass can fog up easily in cold weather), and aid concealment (sunlight glare in telescopic sight lenses can reveal a sniper's position). Another tactic used by Häyhä was to compact the snow in front of him so that the shot would not disturb the snow and reveal his position. He also kept snow in his mouth, so that the vapor of his breath would not give him away.
The Soviets tried several ploys to get rid of him, including counter-snipers and artillery strikes. On March 6, 1940, Häyhä was shot in the lower left jaw by a Russian soldier during combat. The bullet tumbled upon impact and exited his head. He was picked up by fellow soldiers who said "half his head was missing", but he was not dead: he regained consciousness on March 13, the day peace was declared. Shortly after the war Häyhä was promoted from alikersantti (corporal) to Second Lieutenant by Field Marshal Carl Gustaf Emil Mannerheim; no one else has gained rank so quickly in Finland's military history.
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