With a POD after 1900, make it so that a second World War occurs in the 1930s-1940s, with the USA and Britain on one side and Germany and Japan on the other. Germany must control (militarily and/or diplomatically) Europe from the Pyrenees deep into modern Russia at one point, but must still lose.
Bonus points if
-Nuclear weapons are used in the war, but not anytime after
-Italy is allied with Germany
-Russia is communist
-Poland is important in some way
- (just for fun) Winston Churchill (the war correspondent) is Prime Minister of Great Britain at some point during the war.
OK, let's see if I'm getting this right: In addition to the secondary ideas we have three requirements: 1) Germany must have incentive to wage a war of conquest; 2) They must have enough resources to take much of Euriope; and 3) They must still lose in the end.
We start with #1. This would assume that some more radical political entity than the Zentrum/Sozi governments in the '20s and '40s got to power. And this in turn would require some massive threat to scare the masses towards the political extremes. The stockmarket crash might be a start, but that wasn't nearly bad enough to cause anything revolutionary. Perhaps if the Bolsheviks had retained control over Russia, the Rotfront would've been a credible threat with its promises of revolution and a Marxist dictatorship? This manages the "Russia is Communist" aim as well. You could take some early point in the Russian Civil War for that; what if Admiral Kolchak hadn't worked with the Omsk Directorate, for example? Or if Denikin hadn't allied with Pilsudski?
Or conversely, you could have Fascism spread meaningfully outside Italy, perhaps even to Russia; the people there were desperate enough in the famines after the war, the question would be whether a sufficiently strong Party apparatus could be built up. With a Great Power backing them, you could make the German Fascists a credible threat to the Republic.
In any case, this might push the military into taking charge. Such a dictatorship, open or de facto, would be heavily influenced by nationalism and the Right generally, and it might then seek integration with Austria. That would mean trouble with Italy though, so you might look for something else. They might be displeased with the codominium they shared with the French over Elsass-Lothringen after the truce, but I don't think that'd be enough. Perhaps with some high-profiled French provocations, but it still wouldn't be likely. A Russian move on Finland?
In any case, expect close ties to Poland; whatever Russia there is, both will be suspicious of it. Poland may begrudge Germany some of her lands, but that won't get in the way of collective security. Italy is the odd card; depending on whom the Russians support, they might throw in their lot with them or the Germans. If we say that Russia is Red for the sake of the argument, they might find common ground with the German militarists, especially if the pan-Slavic agitation against the areas they snatched from Austria-Hungary when they fell continues.
Well, those are some ideas, at least. As for the war bit, they'd have to strike quickly to take out France before moving towards Russia. They'll not be as outnumbered as one might think, though; A Communist/Fascist government won't do nearly so well in keeping up the impressive economic growth of the Tsar's last decades, and they'll also lose all the great military innovators that developed armoured warfare: Wrangel, Sikorski... In effect, we'll see a scenario similar to the original Schlieffen Plan; for the purposes of this thread, it'll have to be successful this time. With France subdued and the Channel Coast fortified, the Germans could then take on Bolshevik Russia. The Central European nations will have fallen into line by then, and an allied Italy will maintain control in Albania and Greater Slavonia.
Then, everything is set for America to enter the war. How that'd happen I don't know; US history isn't my area of expertise, but you'd have to get rid of Huey Long before his third term for sure. But in any case, their entry will be what spells the end of the German victories. In the end, the Germans might be forced to use atomics to get a decent peace out of the war; if I know them right, it'd be against France.
EDIT: Oh, you wanted Japan in, too... and not on Britain's side? That's almost ASB-ish, though... why would they give up their alliance? Ah, well... I guess there's some expert on Asia who could come up with a plausible POD around...