I'm failing to see how the USSR being a co-belligerent with Nazi Germany goes further than being a temporary co-belligerent. Stalin is never going to sign up to the Axis.
In late Nov. 1940, Stalin had Molotov in Berlin in secret talks with Hitler & Ribbentrop arguing for exactly that. Stalin's conditions were :
[FONT=Arial, sans-serif]1. That German troops evacuate Finland in exchange for Soviet guarantees of continued nickel & wood shipments and peace with Finland;[/FONT]
2. A mutual assistance pact be signed with Bulgaria in the next few months permitting Soviet bases there;
3. The center of Soviet territorial domination would be south of Baku & Batumi;
4. Japanese renunciation of rights to northern Sakhalin oil & coal concessions in exchange for appropriate compensation;
5. Affirmation by Germany that the Soviet-Bulgaria mutual assistance treaty was a political necessity.
You can argue all you like that Stalin wasn't serious & was just looking to get Hitler to lower his guard so Stalin could backstab him. Personally I think Stalin was absolutely serious. He was the biggest dictator of 'em all & he wanted in the dictators' club. Of course, circumstances change & the USSR might have ended up fighting Germany anyway.
But either way, Hitler could have used Stalin's overtures a lot better than he did. Hitler got all pissed over the Bulgaria thing & never answered Stalin's offers, but instead he could have insisted on some deal-sweeteners.
"Our German troops are in Finland only as part of our security set-up to protect the Swedish iron mines & access to Narvik. Besides, what you did to Finland earlier this year has got 'em all worked up. You know how it is-- but they're no threat to you. Hell-- it's only 4 divisions! I'll promise not to increase the garrison, & if you're agreeable on a couple of other things then I might even reduce it."
"The Bulgaria & Japan things-- I can see where you'd have issues along those lines, but it's gonna take some talking. I've got my diplomats out there right now working to see what kind of a deal we can get for you."
"Now your Baku & Batumi idea... I like that a lot. If you were to move towards getting some territorial acquisitions in Turkey & Iran then that'd take a lot of the heat off me from the hard-liners in my gov't. saying what a threat you are. You know how it is. In theory your word is law, but in practice there's all these petty bureaucrats obstructing everything you try to do. But south of Baku & Batumi... Hmmm..."
"Tell you what. I'll work on the Bulgaria & Japan deals. They involve other gov'ts. so they're gonna take time. Meanwhile, you move some military units into the trans-Caucasus region. I'm thinking at least 10 divisions. 15 would be better. Get 'em on the south slope of the mtns. & make some threatening noises that Turkey & Iran can hear. If you'll do that, then I'll reduce the Finland garrison."
"This will put you & me in position for a cooperative invasion of Turkey, in which case I'll guarantee you a port on the Med. Latakia, maybe. We may do it, we may not... Just have to see. And once we get the Bulgaria & Japan things worked out, then we'll get you on-board as a full-fledged Axis partner. How's that sound?"
On another thread I've proposed sending an Axis infantry corps to Iraq in May 1941 to defeat the UK invasion,
...but the fly in the ointment is Turkey. Some of the other posters are convinced that the Turks were a lot more wedded to their neutrality than the historical record supports. But 10 or 15 Russian divisions on their NE borders might change their attitude.
Thegn.