Assuming a negotiated peace between the UK and Nazi Germany in 1940/1941 (with the POD either being Halifax succeeding Chamberlain or the destruction of the BEF at Dunkirk), how much more likely will it be for the Soviets to anticipate Operation Barbarossa? What absurd explanation for eastern military buildups would Hitler have to come up with? And would a greater Soviet alertness have been able to offset the German advantages (less aerial losses to the RAF, no Afrika Korps, no delay of Barbarossa, greater aid from France, Spain; etc.)
I have increasingly come to believe that Stalin was terrified of provoking the Germans, and would have accepted any German excuses in order to justify his delusions, but I'm not at all sure.
The Soviets would definitely have thought a German attack was more likely without Britain distracting German energies. Still, while it would be obvious (just as it was in OTL) that the Germans were likely to attack, the when would still be unclear and the drip-drip of posturing and negotiating temporary diplomatic resolutions would make it easy to think that
this or
that surge in troop numbers was another negotiating ploy, rather than the precursor of an actual attack.
And the Soviets in OTL were kinda hitting alertness fatigue. That could easily happen again in a scenario where the UK had already been defeated. Staying on max alert for months on end... Well, the level of that maximum slowly drops as people get worn out.
I'm not sure that a lack of war with Britain would be enough to butterfly the Balkan campaign. And all told, if the Nazis didn't do a Balkan campaign, I think that would be a net positive for the Soviets. The German flank would be less well anchored, the minor Axis allies that provided vital manpower would be able/willing to provide less (especially Italy, whose importance on the Eastern Front is enormously underrated), the Germans would be less prepared and would attack into less favourable weather. Of course, if Britain is out of the war, all that may not be enough. British support in the early stages of the Great Patriotic war were extremely important for keeping the Soviets on their feet. And while I think it's impossible for the Germans to win in the same year they attack the Soviets, the year after, like OTL's 1942, they could have decent odds of destroying the Soviets as an organized state.
Now, the Japanese managed to do this to Nationalist China in OTL, and it didn't win their war, but so long as the US stays out, its hard to see how the disorganized remnant of the USSR would be more than a nuisance to the Germans as they murdered their way towards Hitler's idea of utopia.
The Soviets could still win this, but no doubt, they are overall in a worse place than OTL. And with Britain already defeated, Japan might be more willing to break their neutrality treaty with the Soviets. Especially since a Britain that makes peace in Europe before Japan launches its offensives in Southeast Asia will have Japan more concerned about biting off more than they can chew if they go with the navy's plan.
What the US does in this scenario would be very interesting.
fasquardon