Why do you, Stellan, think that RKKA was weak? Compared with Wehrmacht?
Do you really think that the officer purges could have weakened RKKA so much? I´ve read a book about the purge: "Pochemu Stalin obezglavil svoyu armiyu?" by Viktor Suvorov aka. Vladimir Rezun. ("Why did Stalin behead his army?"). I presume, you know Russian, though.
The book says it was a fully rational disposition. Stalin wanted his own officer corps, people who owned their promotions to him. He wanted them to be younger and wished to get rid of old bolsheviks, thinking in the categories of civil war.
There were some mistakes: Rokossovsky f. ex. and some others were returned from Gulag, fed and sent back to form new divisions. Just in time to be ready for the big jump on Europe in June-July ´41. Says the book.
Of course, the fact that Hitler started off first was of paramount importance. But for ideological reasons it was impossible to have a big Russian or Ukrainian army. Hitler came there to stay and not to give Russia back to its owners.
Partisan movement: yes, right. The majority of them were trained soldiers sent behind front lines and not a spontaneous movement. Their commander was Voroshilov, right?
Anyways, Hitler believed in one more easy victory. And when it turned out it was a mirage - it was too late. But even then he refused to see the facts in the eyes. Strange man, Hitler.
To make it short: My point is that RKKA was not weak. There is evidence that it was about ready to invade Europe and, imo, fully capable to do so.
I am not saying that Barbarossa was a purely preventive undertaking, but the fear of Soviet attack was one of the most important reasons to strike.
In this light, my hypothesis is that Hitler managed to save ˝ of Europe from becoming Soviet vassals.
Conc. the Japanese: they have learned a lesson in 1939 when Zhukov beat their 6th Army in Mongolia. Japan´s interests were not in USSR, why they signed an non-aggression treaty with USSR in 1940 (afair). Good espionage was also something Stalin was better at than Germans: Richard Sorge supplied evidence of impending Jap. attack on USA. Besides, the US was in itself provocative enough to lead Jap. eyes off USSR.
Are there any objections?