Before World War 1, Finland had been part of Russia. However, during the Russian Civil War that followed, Finland was largely dominated by the non-Soviet White Russians, who successfully broke away from Russian to become their own independent country. This situation was to rankle Stalin, who still regarded the territories of Finland as belonging to his country.
Three months after the outbreak of World War 2, The Soviet Union invaded Finland. Having already made territorial demands of Finland and emboldened by their participation in Nazi Germany’s invasion of Poland, 600,000 Soviet troops crossed the common border with Finland on November 30, 1939, probably assuming such an invasion would be a cakewalk against such a comparatively weak opponent. Some argue that their aim was the ultimate annexation of the entirety of Finland. However, despite the massive flood of invaders, most Soviet troops were ill-equipped and often barely trained.