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The scenario here is that the November 30th advance by Red Army is a complete and utter disaster. The Soviet officer corps (decimated by the Great Purge) mismanages the order of battle so significantly, that the majority of troops/tanks advance directly into the hardest defensive positions of the Mannerheim line near Leningrad, as well as those prepared along the Ladoga-Karelia front.
The political backlash from Moscow sees the sacking (and shooting) of several officers, leading to additional confusion among the ranks. This leads to additional losses and a chance for the Finns to push back, which they do successfully as 1939 closes out. With significant losses mounting with no territory gained, Stalin offers a white peace in early 1940, which Mannerheim accepts.
With Finland retaining its territorial integrity and with Moscow dealing with such a significant defeat, what effect might this have on Europe for the rest of WWII?