The German sprint across Poland in September 1939 introduced the Nazi Blitzkrieg to the world, which then watched as Hitler's forces swept over Europe and bombarded Britain.

Farther north, another battle raged in the unprecedentedly cold winter of 1939-1940, as outnumbered Finnish forces took on the Soviet Union.

The two countries signed a nonaggression treaty in the early 1930s, but that did not allay Finnish concerns about its neighbor. Those fears were justified. The Soviet Union surged across the Karelian Isthmus on November 30, 1939.

About a million Soviet troops crossed into the dense forests and frozen expanses that connected the two countries, but determined Finnish troops turned the bucolic landscape into a charnel house for underprepared, underfed, and overwhelmed Soviet troops.

Finland is thought to have lost about 25,000 soldiers during the 105-day conflict, while the Soviets lost nearly 200,000 troops, with hundreds more stricken by frostbite.

Helsinki eventually succumbed, however, signing a peace pact on Moscow's terms on March 12, 1940 — though Finland did not completely capitulate and would later partner with Germany to fight the USSR.

As the photos below show, the Finnish troops made deft and deadly use of a vicious winter and unforgiving landscape to exact the maximum toll from Russian invaders.