The Soviets never even expected much of a war in the first place. Latter storytelling has embellished the events somewhat, but the essence of what Nikita Krushchev wrote in his memoirs - that the Soviet politbyro expected the Finns to surrender after a token show of force - is apparently accurate.

First of all, the background to the Winter War was the unexpectedly quick defeat of Poland - whose military was considered quite strong before 1939 - under mechanised Nazi attack and Soviet stab in the back, and quick and bloodless occupation of Baltic countries (which had acquiesced to the same ultimatum presented to the Finns) by Soviet forces. Against this background, everyone in the world except the Finns had every reason to believe that a war between a poor, armaments-deficient country of 3.5 million and a technologically advanced military power of 150 million people should not even be called a "war," but rather a police action or some such. The Communist Party secretary for Leningrad district, Andrei Zdanov, managed to convince Stalin that uppity Finland would be put into her place using nothing more than well-equipped mechanised troops from the Leningrad military district; in this assessment, the Soviet leadership was undoubtedly further persuaded by the beliefs of Finnish expatriate Communists, who had fled the to the USSR after the 1918 civil war and either believed, or at least found it more conducive to staying alive to claim to believe, that Finland was still deeply divided between the rich Whites and the poor Reds, and that the proletariat would welcome the Soviets as liberators or even rise in another rebellion if the war began. To put this mildly, these beliefs proved inaccurate: in fact, dozens of former Red Guards, some of whom had received a death sentence in 1918 (since commuted) from the Finnish Government, tried to volunteer to the front to fight off the Soviets.

Furthermore, Stalin had little reason to fear anyone aiding the Finns: even if the war protracted, the Brits were at war with Germany and therefore the Baltic was hostile to British shipping, the Germans were bound by the Molotov-Ribbentrop pact (which carved Finland and the Baltics to the Soviet Union's sphere of influence), and the small Nordic neighbors were to be pressured to neutrality. By moving fast, Stalin would be able to capitalise on the situation before any help could be arranged and before the Molotov-Ribbentrop pact ran its course, so he had an element of hurry influencing his decisions as well.

Finally, the military campaign against Finland was originally planned and commanded by one of Stalin's more incompetent flunkies, general Meretskov, who too thought the war would be a cakewalk that would be over in two or three weeks, even as some of his subordinates provided more realistic assessments and believed the war would take at least three months to finish. (It took 105 days in reality, so these folks probably had a fair grasp of their profession.) These assessments were discarded and the subordinates were reportedly lucky to not be shot for spreading defeatism.

Therefore, it is no wonder that the Soviet advance was spearheaded by marching bands in full dress uniforms and among the loot captured by the Finns there were, for example, flags that were to be used in a victory parade in Helsinki and strict instructions as to how the Soviet soldiers should behave towards the Swedish border guards.

Finally, it should be noted that the winter of 1939/40 was one of the coldest in living memory. In any other winter of the decade, the Soviets would probably have had much less trouble with the weather.