I would disagree fundamentally with the assertion made in this thead that the Soviets would basically fall over.
First, the air. On the morning of June 22, the Soviets had roughly 8300 combat-ready aircraft, compared to 2600 German aircraft, a ratio of around 3:1. However, this is misleading because of obvious problems with the Soviets’ doctrine. Firstly, these aircraft were attached directly to Soviet field armies and thus suffered from a similar problem to Allied armour a year before. Soviet planners had drawn erroneous conclusions from the Winter War and believed that air forces should be subordinated to the armies they supported, since against Finland Soviet air had been unable to coordinate effectively with ground forces. However, this meant in practice that even worse coordination problems were experienced, since the labyrinthine structure of the air forces prevented their effective deployment. Without a central body effectively directing air operations the Soviet aircraft were unable to bring their full strength against German aircraft when needed. Additionally, Soviet bombers were rendered ineffective due to the inability of the Soviet commanders to provide fighter escorts.
Secondly, the size of the Soviet air forces paradoxically reduced their efficiency, as the Soviet officer corps was simply not sufficient for the number of formations. The Air Force was one of the branches of service hit most heavily by Stalin’s purges, meaning that the size increases effectively rendered it a discordant mass of aircraft.
It’s hard to estimate exactly how this effected the usefulness of a given number of Soviet aircraft versus their German counterparts, but I would argue that, despite the gross ineffectiveness of the Soviet air, it was still a potentially dangerous force. Which brings us to our third point.
I consider the surprise achieved by the Axis invasion to be the decisive factor in Barbarossa, and nowhere is this more true than concerning aircraft. Of those 8300 front line aircraft, 2000 were destroyed in the first day of combat, most of those on the ground, reducing the Soviet numerical superiority in the air to 2.4:1. The key phrase here is on the ground. Soviets did not anticipate the scale or suddenness of the attack; the Soviet leadership effectively denied its very possibility. The Germans dropped thousands of tons of bombs and with the core of their air power gone the Soviet losses increased from there. Tellingly, during the time from June to November, 5,000 Soviet aircraft simply vanished from Soviet records, an eighth of all the Soviet aircraft deployed in the campaign. The Soviets had no plans after much of their their air forces had been wiped out and tactical developments simply escaped them.
All of this goes away if we assume Britain peaces out. Now the Soviet leadership knows that the only next target for Germany is them, so obviously they will begin preparing in some way. OTL they could not bring themselves to believe that Hitker would subject himself to the mistake that had doomed his country in the last war and commit to two fronts, but now they know he has no reason to not attack. So Soviet air forces on the front will be put on alert if anything happens. The Soviets did have warning signs for Barbarossa OTL (you can’t exactly move 3 million men into position without the other side of the border noticing something, after all) but ignored them. If that happens here it will set off alarm bells at every level of Soviet leadership and command.
So now Soviet aircraft will not be in their hangers when the German bonbs fall, completely changing the air war. Soviet air will be subject to the same disabilities they already had, but as the Manchurian Strategic Offensive Operation, Okinawa, and Stalingrad demonstrate, tenacity (which was at least something the Soviets were not deficient in) can produce high casualties on the opposing side, which the numerically inferior Germans cannot sustain.
Armies. Like the aircraft above them, Soviet troops will be placed under incompetent commanders following flawed doctrines, but now those commanders will benefit from a good appraisal of the situation. OTL Soviets were given orders showing a total misunderstanding of the invasion. As I mentioned above, Soviets did not anticipate the scale or suddenness of the attack, and they ordered massive counterattacks that resulted in crippling casualties. It’s probable that these will still occur here, but now Soviet divisions will be in position to at least try to carry them out effectively. German invasion forces will have a nasty shock as they plow into something almost approaching an effective counteroffensive. Even so, these attacks are doomed to fail, and the Germans will still occupy large areas of the western Soviet Union. But their gains will not be so rapid, and Soviet victories not so isolated.
To be clear, I am not suggesting that Barbarossa would fail totally. Germans simply had too much concentrated attack power that cannot be deflected, especially with the reinforcements from the west. Keep in mind though that German forces in France 1942-1944 were poor-quality and wouldn’t add much to Barbarossa. The Afrika Korps was more elite, but OTL the Germans spared the minimum possible effort in the Med, and their numbers would be small. Although lend-lease obviously won’t be available here, that only began to have an effect from late 1942 onwards, by which time the Soviets had already turned the tide on their own.
There are two more thing I would like the mention. First, peace in the west would mean an extended period in which Germany was not technically at war. This is a false positive to Hitler (who OTL in early 1941 actually suggested that several entire divisions be sent to do agricultural labor) who will probably compromise Germany’s “war” economy, such as it already was. Yes, I’m serious about this. It’s a well established fact that the Nazi leadership had absolutely no idea as to how to run a nation at war (for example German war production peaked in 1944 for reasons that would never had occurred that late in the war had the Nazis had any economic sense at all).
Second, as has already been mentioned in this thread, it’s possible that Hitler originally scheduled Barbarossa for 1943 or 1942, which will give the advantage to the Soviets given the track record of tank development and the damage Hitler will probably manage to do to the Wehrmacht in that time.