
Note also that significant neutral powers such as Sweden and Spain were economically within the Axis sphere
Observers generally agree that Hitler's biggest error in World War II was attacking the Soviet Union and exposing Germany to a 2-front war. It didn't work out so well, after all.
But it's often pointed out that a war between the two great European dictators was inevitable, after all, and Hitler had little choice but to settle things one way or the other. True, I think, but also missing the point. It wasn't the question of IF, but of WHEN. Part of the reason why Stalin was caught by surprise was that he judged that the Germans weren't likely to start the war so soon. And Stalin was probably right.
While Hitler was evidently trying to capitalize on a known German advantage in quality while it lasted, I see little reason to think that advantage was in any immediate danger of narrowing. Indeed, more attention to war preparedness on the German side and an earlier start to the campaign season (undistracted by Balkan adventures) in 1942 would probably have seen very similar results to the 1941 campaign -- but with another 6-8 weeks of campaigning. Sure, the T-34s and KV-1s would have been a nasty surprise, but German successes had never depended on superior tanks anyway and there's every reason to think that the Russian tank crews would have been unable to translate their better tanks into tactical success.
A delay in the start of Barbarossa until 1942 would have had some other interesting implications. For one, it would have meant Britain fighting on alone for yet another year. Undoubtedly, without the need to immediately divert resources for the upcoming Russian campaign, events in the Middle East and the Med (Greece, Crete, Iraq, Western Desert) could have easily taken a very grim turn for the Allies without requiring all that much in extra German effort. Another Panzer division or two, a Luftwaffe air group and it's easy to see the Germans overwhelming the overstretched British.
What about the Americans? Well, the trajectory of events driving the USA and Japan to war was largely independent of the Russo-German question. It's probable that Japan and the US would have gone to war more or less on the historical schedule and it's an open question whether Hitler would have repeated his historical mistake of gratuitously declaring war on the US (he wasn't required to by treaty because Japan had attacked the US) . If he didn't, it's hard to see how FDR could have even brought up declaring war on the Germans when it was Japan that had attacked, let alone following any kind of "Germany First" strategy. The natural course of events would have focused American attention on the Pacific first -- bad for Japan but a boon for Hitler. Undoubtedly the Russians and Japanese would have followed their historical course and avoided hostilities for now.
So instead of merging into one grand Second World War in 1941, we may have seen two large regional wars instead. A sputtering European/ expanded Middle Eastern theater and something similar to the historical Pacific War. There would have been a slight overlap in combatants, as the British had their Far East interests, but they may have been unable to make even a token defense given the crisis elsewhere.
A more patient Hitler, striking Russia in the spring of 1942, would have had a much more favorable strategic situation to deal with than he had in 1942. Britain, still isolated, would have beset by even more foes and without as strong help from the USA as it historically received. America would have been emotionally and logistically well and truly committed to the Pacific War and unlikely to change course in mid-stream and Russia, with the training, doctrine and leadership deficiencies from the year before still endemic, would have still been vulnrable to being blitzed into defeat.
Of course, Hitler was not known for his patience, but he could muster it on occasion. And it's hard to see what urgent requirement was driving the need to start the Russo-German War in 1941, rather than waiting less than a year more.