Some people think that the reasons for World War 2 are embedded into World War 1 and in particular in its "strange" end.
The majority of Germans at that time (1918) didn't accept or understand the defeat of their country and never accepted it assuming that it was a kind of big treason by some kind of internal obscure force (for example, the Jews).
The economy also was shattered (high inflation and unemployment) and the political situation dangerously oscillating between democracy and dictatorship (on one side you had democratic forces and the communists on the other the war veterans and nationalists ferociously anti-communist fighting each others in the streets).
Also the German resented the terrible war reparation costs they had to pay to the victors and the fact that some part of their country were given to other powers (Alsace and Lorraine to the French, for example).
From this difficult situation emerged Adolf Hitler and the Nazi Party. They managed (most of the time using violent means) to establish order (politically and socially) and give Germany a new prosperous economy and new pride (focussed upon the creation of a new and modern army).
But Hitler, allegedly, needed to expand to get vital space and resources (and probably, secretly, wanted to avenge the humiliation of World War 1). He started to invade small European countries (Austria, Sudetenland, Czechoslovakia) and at the end tried to bite the big chunk, Poland. Germany at the time (1939) was divided from its northern province of Prussia (east) by a corridor of Polish administered land (the Danzig Corridor) basically dividing Germany and Prussia.
With an excuse Germany attacked Poland but, similar to the start of World War 1, it started a mechanism of defensive alliances; Poland was an ally of England that in turn, was an ally of France. England immediately declared war on Germany to support Poland and in doing so pushed France to help her against the Germans and declare war on Germany as well.
It is necessary to say that if the treatment of Germany after the war had been more lenient probably it would have avoided its embitterment and fall towards Nazism.
The reasons of World War 2 are, in my opinion, to be found in the terrible situation of Germany after World War 1 and the need for the Nazi regime to maintain its control on the nation by embarking into a war against the same nations that defeated Germany before.