How could WWII developed if neither USA nor the USSR would have been involved?
Like:
- Nazi Germany never tries to invade USSR and respects the Molotov-Ribbentrop agreement (and also the USSR, of course).
- Japan never attacks the US and the isolationist policies prevail in Wahington DC.
I guess the UK alone would have hard time to pull a continental invasion against Nazi Germany, but in the other hand it would be also complicated for Nazi Germany to keep the mid-war status (before Barbarossa) stable for long.
Can I first ask why you chose not to look up any of the dozens of threads that ask the same question? As to the question itself its all but impossible for Germany to avoid conflict with the USSR. leaving aside Nazi ideology for a moment after the Summer of 1940 Nazi Germany controls the whole of Western Europe and promptly sees a collapse in industrial and agricultural output as it is cut off from foreign imports, so the only way to exploit the base efficiently is with food and raw materials from the USSR. Sure under the M-R pact the Soviets are willing to supply them, but that not only makes Nazi Germany utterly dependent on the USSR they are also having to trade valuable technology that ultimately strengthens the USSR's war machine. Ultimately Nazi Germany faces either subservience to Moscow or it seizes control of those resources for itself. Turning back to ideology of course there was no way Nazi Germany and the USSR could co-exist, the M-R Pact was little more than a postponement of hostilities with both sides knowing that war was inevitable sooner or later.
As to the USA isolationist sentiment is often overstated, yes the voters in the USA were against getting dragged into fighting the war but supporting Britain was another matter, remember Roosevelt was no dictator, he had to get Congress to support his measures and reluctant as they may have been they progressively approved ever greater US support for Britain, not out of altruism but because they recognized the strategic nightmare that would face the USA if Britain were to fall under Nazi influence and the Royal Navy was at Hitler's command. As for the Pacific Japan in 1941 needs resources as badly as Nazi Germany and the only place they can get them is in the so-called 'Southern Area', which will inevitably mean escalating tensions with the USA, which might be reluctant to confront Nazi Germany but is altogether more proactive in the Pacific.
Short version, to keep the USA and USSR out of the war you are going to have to offer up some fairly major PODs and honestly few if any of the previous threads on the same topic have offered up anything convincing.