Schumpeter (and Arno Mayer) would say that the persistence of the Old Regime into the age of industry brought about the biggest wars of all time. I think they're right about World War I.
I still don't understand the causes of the Nazi seizure of power and the origins of World War II well enough. Lots of people were eager to knife the Weimar Republic in the belief that they would pick up the pieces. Right now my list of causes of World War II is, roughly in order:
(1) Hitler
(2) The Great Depression that undermined support for Weimar (and the rapid recovery from which in Germany solidified Hitler's power base)
(3) German rightists who wanted a strong, militarily aggressive, anti-Western and anti-Bolshevik country
(4) German industrialists who thought that a populist demagogue with right-wing leanings would weaken the left
(5) The Comintern that thought that it was destined to pick up the pieces if only Weimar could be smashed
(6) British and French politicians who failed to realize that they had as big a stake in the success of the Weimar Republic as anyone
(7) The strain of "reactionary modernism"--as Jeff Herf puts it--in German culture
But in my view at least the simple Neumann-Behemoth-Poulantzes fascism-is-the-highest-state-of-capitalism interpretation doesn't even work for fascism--and Nazism is something very different.