The trouble is that we consider Nazi Germany in need of regime change based upon what we know they ended up doing and judging that by today's standards. When exactly do you think the Allied nations would have been justified in pre-emptively attacking Germany? Until Hitler took over Bohemia and Moravia in March, 1939, he hadn't taken over any territories except ones with a clear German majority, the population of which subsequently voted overwhelmingly to incorporation into the Reich.

A lot of people today claim that everyone should have known what Hitler was going to do in relation to lebensraum and the Holocaust but in fact few politicians carry through on their more extreme rhetorical boasts. Yes, there were warning signs from the Nazis but the actual carrying out of the Holocaust surprised even most Germans.

So, in this light, it is not clear that the Allies should have openly opposed Hitler any earlier than they actually did. This leaves your argument about an unnecessary war hinging on the Germans themselves. But how are we to force the Germans to not accept or throw out the Nazis. In the 1930s the Nazis in power were incredibly successful, at least on the surface. This is why the Nazis went from not having even a majority of Germans voting them into power into receiving overwhelming plebiscite victories. The trouble is that there was far more a militaristic presence in Germany than in most other countries, and the way WWI ended, without Germany being clearly defeated on the field of battle, allowed the myth of the dolchstoss to lead into not only support for renewed militarism, but xenophobia against "foreign" elements in the German nation. Considering the way that the First World War ended I see little other way things could have turned out but WWII. Without the Nazis the war probably wouldn't have occurred as quickly but the militarist element in Germany was still much better organized and fanatical than the liberal element.

Jaeger talks about democracy not having been given a fighting chance in Weimer Germany. The fact is that it didn't have a chance because few Germans had any respect for democracy and associated it with the humiliating defeat in WWI. Those liberals who wanted democracy were few in number and were more interested in enjoying their newfound freedoms than in safeguarding those freedoms and so came to lose those freedoms to the extremist elements. Democracy in such a situation naturally lead to an extremist party getting into power and that led into WWII. So, barring a change in the ending of WWI I don't see how WWII was an unnecessary war.