While Finland isn't seen as a probably aggressor from the beginning of an attack on the Soviet Union, the country is nevertheless suspected of potentially wanting to join the Germans as soon as it would appear that the invasion became successful. The Soviets also lumped Finland together with a number of East European states which, at the time, had dictatorial or at least semi-dictatorial governments with more than just a streak of Fascism (I wonder why Lithuania isn't mentioned, but we'll let that rest for now).The Soviet Union's assessment of the threat to its territory remained quite constant throughout the late 1930s, and the operative plan for 1939 began on the assumption that Germany and Poland would attack the Soviet Union simultaneously from the west - with the probable participation of the Italian navy - while Japan would attack from the east. It was estimated that Estonia, Latvia, Romania, Bulgaria, Turkey and Finland would join the war only if the operations of the Red Army and the Red Navy did not meet the success from the outset.
This seems highly surprising. Finland in 1939 was a democratic state with strong left-wing leanings. The biggest party was the Social Democrats, gaining in general some 40 pct. of votes at elections. Of course, there is a vast difference between Social Democrats and pro-Soviet Communists, but the thought of a country like this jumping on the Nazi bandwagon in an invasion of the Soviet Union seems positively absurd. We know how difficult it was to carry though such a project in Finland even after the Winter War when political and public support for it was much higher. Furthermore, we know that the Finnish army in 1939 was relatively small, under-equipped and geared mainly for defense, and though there was a long-standing historical bond between Finland and Germany, that bond had become considerable less important during the 1920s and 1930s, when Finland turned more towards Britain and, of course, the other Nordic countries while engaging strongly in the work of the League of Nations.
Still, it was this suspicion of an attack from Finland aimed at primarily Leningrad that led to the Soviet demands of a border adjustment in the autumn of 1939 and subsequently to the Winter War.
So how and why did the Soviets end up with this false impression of Finland, leading to so much misery on both sides?