Well I'm pretty confident the USSR would lose
Not quite so sure about that. But first my premise.
Stalin is forced to become more aggressive beginning in March 1936 when the Rheinland is reoccupied without opposition by the French. There is a spontaneous outpouring of of sentiment by military and civilian segment of Soviet Society that Hitler must be stopped, and the USSR will inevitably be forced to act alone. Although Stalinist purges commence, they quickly lose credibility when a number of those placed on show trial are the loudest in demanding war with Nazi Germany. Stalin's terror grip is loosened.
Hitler's patience is forced in September 1938, when prior to Munich the British and French begin mobilization, and at the Four-Power Meeting there coldly inform Hitler that Czechoslovakia is inviolate, a state of war will exist if German forces cross the border. As Hitler continues to pressure, German generals shrink from the prospect of war. In rapid succession, generals, including key divisional commanders claim Hitler has broken his oath to the German State, and therefore they are released from their own Fuhrereid. The balance of power shifts in favour of the German Armed Forces.
Despite this, in March 1939, the Germans force the Lithuanians to return Memelland. Stalin responds by forcing Lithuania, Latvia, and Estonia to accept Soviet troops on their soil the following month.
I'll call this The Premise, and allow others to chirp in freely on its plausibility.
In August 1939, Stalin demands that Finland and Poland also allow the stationing of Soviet troops on their territory, and both refuse. On 1 September 1939, the Soviets cross the borders of both nations, and Germany instantaneously declares war. Poland, much to their dismay, are forced to have Hitler as an ally. Two days later both Great Britain and France declare war, but neither exerts much effort than sending expeditionary forces to Finland to restore its territorial integrity and perhaps limited gains such as the capture of Leningrad, or liberation of the Baltic States. They certainly won't accept the risks of mass casualties a march on Moscow entail. Nowhere else do British or French territories border the USSR. London and Paris will break out the hors d'oeuvres and Irish Whiskey and watch the German and Russian casualties mount. Japan will certainly not end its war in China in favour of a conquest of Siberia; it will limit the efforts of the Kwangtung Army to border adjustments around Manchuria.
I call this The Outcome, and offer it for the same critique.