With a PoD after the Nazi invasion of Poland, have the Wehrmacht successfully invade and even defeat the USSR, but have the Axis Powers eventually be defeated, before 1950 and without any atomic bombs.
Not that hard, have a few things go their way during June-July (I can suggest a couple of things) and Leningrad falls in July, which opens up a lot of butterflies that would not benefit the USSR. Have some of those result in the fall of Moscow due to an earlier offensive against the city and a subsequent failure of a Soviet counteroffensive in the autumn/winter of 1941-42 and the USSR beings to unravel. 1942 still sees a major offensive on the Eastern Front to finish off the USSR, which forces an Wallied Autumn invasion of France, the plan to occupy the Cotentin Peninsula to build up a redoubt rather than go full break out in 1942 or early 1943. Have that hang on, which actually shouldn't be that hard given the floating artillery park the Wallies had as well as their air force replacement abilities, and eventually start grinding it's way out of the bridgehead over the course of 1943-44; since a huge part of the German army would still be needed in the East and in the meantime the Wallies could also launch an invasion of North Africa and later Italy while German attention is glued to France, that is certainly do-able without Soviet level casualties being suffered by the Wallies, though they will suffer quite a bit more than IOTL. The strategic air war will be where things are won, because once the Luftwaffe is ground down like IOTL bombing of industry, transportation, and oil will happen and collapse the German economy. It may well take through 1945, but it will happen.
The problem is no nukes once you get into August 1945. Perhaps ITTL the Wallies devote less resources to the program due to having to build up a bigger army to make up for the lack of Soviet help beyond 1943, so there a delay in the final product? But once the bomb is available the Wallies will find somewhere to get a combat test of it before the war ends. It is possible to potentially get a German surrender before that happens, say if Hitler is successfully killed in an assassination and the resulting civil war collapsing the German army's ability to resist in the field, but who knows if that is likely or not.
What would such a world look like? Would Russia and Eastern Europe have been governed democratically? Could this defeated Germany escape an alt-Morgenthau Plan? How would Communism be viewed across the world, had it collapsed after the german invasion? What would the borders of Europe look like, what would societies and politics of the post war world be like, with the two great totalitarian regimes of the 20th century being completely destroyed? Might we see earlier “end of history” style remarks by academics and intellectuals? Would populism rise in such a world? Could social democrats still implement Keynesian policies, would here be the internationalist spirit that the world was founded on?
I'd say a lot would depend on what happens to Russia and Eastern Europe in the meantime. If the Wallies get units there they would favor non-communist governments and since the USSR is the only are that had it's own communist government without external support imposing it in some capacity I think it is likely that everyone gets a US supported 'democratic' government. Communism would likely be a non-entity without the USSR surviving the war and viewed as an ultimately failed system that couldn't stand up to Fascism. I don't think TTL's Germany would escape some sort of a Morgenthau-lite program, but they are still too important to the European economy to simply Morgenthau. Plus without FDR alive past April 1945 there wouldn't likely be the institutional support for such a plan, even with a more damaged world due to a more successful invasion of the USSR. But there would be a lot more punishment of Germany than IOTL, at least by Wallied standards.
I'd say that a social democracy type movement in Europe may or may not be possible depending on the level of destruction and population death, but without the Cold War there is more money for social spending. Of course without an ideological foe to resist the world might go full capitalism earlier and harder with all the resulting inequalities that would result. Its really hard to say given the butterflies.
