Stalin knew the Red army was not ready for war in 1939, hence the Nazi-Soviet Pact. His miscalculation was that he thought it would buy him time until 1942.
Stalin was being courted by France and Britain as well as Germany in August 1939. Stalin thought the allies wanted sit back and do nothing, and defeat Hitler at the cost of copious amounts of Russian blood. Stalin wanted no part of this and cut his deal with Hitler. He thought the Germans, British and French would bleed themselves out again just like World War 1, and he’d clean up in Europe when it was done.
But he miscalculated. The French got KO’s in first round, threw in the towel and the Brits hid behind their moat. That left Stalin to face Hitler, alone, with an army he knew wasn’t ready for war, since he’d killed most of it’s officers and scared to death all the rest.
So the way it worked out was that the British sat it out, and defeated Hitler at the cost of copious amounts of Russian blood.
I love history’s ironies.