What was the most important cause of the Second World War? Obviously there was the Treaty of Versailles which played a significant part; the War Guilt Clause, Reparations, Disarmament and territorial clauses, etc. But, what about Hitler's actions? The failure of appeasement and the failure of the League of Nations?
The most important cause of WW2 (as of WW1) was imperialism.
By that, I mean a specific development of capitalism that features concentration of capital which has enough influence in state affairs to dictate expansionist policies in its favour.
As a result, nations struggle to expand their spheres of interest, i.e. access to markets and resources, which will lead to war as soon as there are no ‘free’ markets left (cf Boxer war – unity amongst European powers to open a fresh market) and opposing power blocs have formed (various European crises in the early 20th century did not result in a war because involved great powers were not sure enough their allies would support them in a war, power blocs were not formed).
Not surprisingly it was Germany who stroke first to challenge its rivals (1914) – after all, France and Britain had established their colonial empires long before Germany was united and became Europe’s strongest industrial power, however with limited access to markets and resources compared to UK and F. However, WWI failed to resolve the tensions. Germany was beaten, but after the economic crises in the early 20th century her economy still proved to be superior in comparison to her direct competitors. Furthermore, due to the revolutions, Germany was also not occupied or disarmed. So it is no surprise, that with the economic power base and imperialist ambitions still in place, German imperialism would surface again to challenge the unprivileged position in the international system imposed by the Versailles treaty.
In my eyes it is wrong to reduce Germany’s war ambitions to revanchism due to Versailles. After all, the other main challenger of the status quo, Japan, was never disadvantaged in any comparable way. Japan started the war due to imperialist motives – to conquer China and Oceania for their resources and manpower.
Neither the treaty of Versailles nor the failure of the League of Nations are causes for the Second Word war. These events are just consequences of imperialistic brinkmanship.
Suggested reading: Mandel, The Meaning of the Second World War
I think the cause was the same as of the WW I: German militarism and expansionism. Since Kaiser Wilhelm gained power in Germany, it has been pursuing an aggressive foreign policy (e.g. the Morocco crisis) and launched an arms race with Great Britain. It resulted in one world war, which did not prove conclusively to Germans that militarism doesn't work, so they launched another one 21 years later.
Read the book "Dreadnought" by R.K. Massie. It describes the German sentiments in great detail. You'll see that Hitler's rhetoric was nothing new in German politics, at least qualitatively. (except for the genocide part, but even that just barely) He was more rabid than, say, Bulow, but the substance was the same.
I'd argue that the biggest reason the war began is that other nations did not do enough, if anything, to prevent it. The Allies of course were not ready economically or militarily for conflict, until they finally took action after Poland. The League of Nations, while a good idea on paper, was also ineffectual against Germany in Europe and Italy in Africa.
Surely the Allies/LN could have seen what was happening and where events would lead. There were attempts by insiders to warn outside powers of what was coming, but to no avail. Hitler's rise to power in Germany could not have been missed. It seems like the Allies were just really, really hoping that nothing bad would happen.
Yes, I agree with all of your reasons, but I'd say that the Treaty of Versailles was the biggest cause. If you listen to Hitlers speeches, they are all about how evil the other nations are for making Germany this weak. (along with some racist statements) I've heard it argued that Hitler just wanted power and used public opinion to his advantage, but I'd argue that he truly believed in what he said because even when the Nazi party was being destroyed by larger parties, he stayed with the Nazis. Hope that helped.