The usual focus of speculation on the Franco/Soviet alliance is on the military aspects, a counting of divisions, arguments over Red Army fitness for combat, and inevitably remarks how the Poles would never allow the Red Army on their territory. (They did refuse the entry of the German army in Septmber 1939, how successful was that?)
But the Soviet contribution need not include major invasions to contribute significantly. In other discussions its argued how the German/Soviet treaty of 1939 enabled Germany to mitigate the Anglo/French blockade and bought Germany a year or two of economic time to continue the war in the west. If all that has any validity then the USSR at war with Germany in September 1939 is the converse. No grain imports, no raw materials, and the USSR has the ability to leverage other adjacent nations into further cooperation with the blockade. Any oil from Rumania ceases, ores from Turkey, the same from Finnland, my numbers are back of the envelope, but they look serious for Germany in 1939-40 if not catastrophic. All that without the need for full mobilization & gargantuan battles in the east. If the Red Army must get to grips with the Germans the low budget route is via Lithuania vs East Prussia. This token military effort would be politically difficult for the nazi regime. East Prussia is a strategic dead end in most terms, but not defending it is politically catastrophic for the nazi regime. Success there also out flanks the Poles & places greater pressure on them to cooperate with the Allies.
With or without a Prussia campaign extending the Blockade entirely around Germany may lead directly to economic collapse in 1940, vs two years later in 1942 as had been calculated with the Soviet/German treaty of Brest Litovisk & Soviet support of Germany.