I know it's quite far-fetched. But Stalin was not always predictable. Let's imagine that he had started to have misgivings about his treaty with Germany at the end of the Winter War, and had secretly ordered the Stavka to make contingency plans in March 1940. So on May 10, when Germany starts its great offensive on the Western Front, he decides to launch an invasion against its former ally. Would had it been possible? Would had it succeeded? The Red Army was in bad shape in the spring of 1940, however it was huge. It would have taken many weeks to mobilize it, especially with the mobilization proceeding discreetly so the Germans don't become alarmed. Surely at the beginning Stalin would have believed to have all the time he needed, because he would not have imagined that the Western Allies would collapse so quickly. If he had waited until June probably it would have been too late, the Germans would have had time to finish the French and move their forces to the East - like they had planned to do in 1914. They had over 40 infantry divisions in reserve when they launched Fall Gelb, and those forces could have been used to delay the Red Army. By May 20 or so however it was clear that the W. Allies were crumbling. Could Stalin have launched an offensive with the forces at hand before the end of May? What do you think would had happened if he did?