Given the state of conflicting ambitions in Central
OK, let's start off with, yes there have been lots of no Nazis and everyone's happy TL's.
Edit. Wrong button.
Let's say it again. No Nazis. IMHO there would have been a war in Europe in the 1940's with who ever is in charge in Germany, or at least a European involving war USSR.
OTL up until say 1921. Nazis still about or never get of the ground, they don't matter for this question, or very minor if they do. Who comes to power and when, where and with whom will German fight? Just France? France and the UK? Just USSR, will they fight because of just the injustice of Versailles or something else? Or does it not just happen at all because of no Hitler? If you know what I mean.
Type about this for a while.
Given the state of conflicting ambitions in Central and Eastern Europe between what amounted to two major powers no matter
who was in charge in Berlin and Moscow, and a post-WWI/Russian Civil War POD in 1921, yes, odds are
high for a major war in Europe at some point in the 1930s-40s.
There's also the possibility of a Russo-German entente to divvy things up, and then a mutual turn toward the peripheries - Scandinavia for the Germans, the Balkans for the Russians, potentially.
Given the strategic realities that led to the Little Entente, historically, the French are likely to become involved in the same "Great Game in Europe" potential conflict(s), and there's always some possibility of a Balkan/Mediterranean littoral conflict growing large, as well. Add it together, and it pretty much is a clear path to a general war in Europe. There's also the (remote, but possible) possibility Franco-German alliance, as well. Depends a lot on events in post-WW I France, of course.
If things go badly in Central and Eastern Europe in the 1920s, and France opposes, the question of whether Britain joins with the French against the Germans (or a Russo-German alliance) is an open question, but given Britain's historic policy of opposing Continental hegemons (real and potential) certainly is in the realm of the possible; this, of course, opens the potential for Italian and/or Japanese involvement (as historical) based on the British and French being otherwise occupied.
The biggest open question is the US; a European war that leads to a hegemonic power in Eurasia is a significant problem for the US by the 1940s, much more so then the same would have been in the 'teens, for obvious reasons...
But the result could be a cold war between the US and the Eurasian power, with US support for whatever potential proxies are left in Europe - again, not unlike what happened historically in the short of war period in 1940-41.
Whether that actually leads to an Atlantic war, or is simply contained by a Blue-Black cold war (or Blue-Red, if that's the way Europe goes) is an interesting question.
Best,