Having written in the previous post about Stalin being unable wage war in 1939, I wanted to write about the other factors in his decision. Stalin in Sept 1939 was faced with the following options:
1) Spurn Hitlers offer of a Nazi-Soviet Non-Aggression pact
2) Accept Hitlers offer
3) Reject Hitlers offer and join a Soviet/British/French alliance.
Option 1 makes no sense. It would have allowed Germany to conquer all of Poland and also establish bases in the Baltic states. It also would have left the USSR with no allies and open to a German attack.
Option 3 was rejected for the following extremely valid reasons. Stalin had the best spy service in the world. Communist spies had infiltrated the French, English, and even German governments. Stalin had an excellent idea of the military strength of all three and of their future war plans. Stalin knew Hitler was intent on attacking Poland. A key reason for a Soviet/Allied alliance would have been the defense of Poland which would have involved Stalin in a war he was in no position to fight. And He knew the French only planned to stay behind the Maginot line and the UK had only a couple of divisions. Any war would be fought by Stalin.
So allied alliance provided Stalin with a lot of risk and very little reward. The British/French had no little ability to tie down the German army - so Stalin would end up fighting Hitler almost alone. Finally, even if Stalin was to win or hang on, the chances of England or France allowing the Red Army to "liberate" Poland or Germany were slight. Further, Stalin could not trust the French/British not to make a compromise peace with Hitler or the German Army
Option 2 was the best of the three. It meant the allies and Germany would be involved in a long and probably drawn out war. During this time Stalin could continue to rearm and build up the Soviet Military and war industries. Stalin would be able to occupy and "liberate" the Baltic nations, part of Romania and Poland, and possibly Finland. Should the Allied-Germany war end in a Stalemate Stalin would still have his territorial gains. Should the allies win, the situation after WW I would be replayed with a possible communist revolution in Germany and a Red Army occupation of Poland. In fact, had the Germans been on the verge of losing to France/Britain Stalin could have then attacked Germany and gotten Poland and part of Germany. The only risk to option 3, and the least likely, was the one that actually happened. A relatively quick German victory over France. Yet, even this unforeseen outcome enabled the Soviets 2 years to build the largest army in Europe and eventually defeat Nazi Germany.
Given what we know the Soviet build up Stalin had every reason to believe the Red Army in June 1941 could either defeat or win a draw against Hitler. The German army in June 1941 had no superiority in numbers of tanks or men. And once fully mobilized the Red Army would have 12 million men compared to the Wehrmacht's 8 million. Further with a 35 million men
of military age to Germany's 17 million, and with equal numbers of tanks and aircraft; Stalin had every reason to believe in ultimate victory.
Like Lenin's treaty with Germany in 1917, the treaty was with Hitler was a masterstroke that not only allowed the USSR to survive but almost ended with all of Europe going communist.