The Winter War — William Durbin

When the Soviet Union invades its tiny neighbor Finland in November 1939, Marko volunteers to help the war effort. Even though his leg was weakened by polio, he can ski well, and he becomes a messenger on the front line, skiing in white camouflage through the forests at night. The dark forest is terrifying, and so are the odds against the Finns: the Russians have 4 times as many soldiers and 30 times as many planes. They have 3000 tanks, while the Finns have 30. But a tank is no help in the snowy forest -- a boy on skis is. And the Russians don't know winter the way the Finns do, or what tough guerrilla warriors the Finns are. Marko teams up with another messenger, Karl. Gradually Marko learns that Karl's whole family was killed by the Russians. And Karl has a secret -- he's really Kaari, a girl who joined up to get revenge for her family's deaths.