Hitler had indeed already written about some of his grand ambitions for Germany in the chapter Eastern Orientation or Eastern Policy (chapter 14 in volume 2) in his autobiography Mein Kampf. This chapter specifically discusses Lebensraum, which guided the mindset and policies that led to the invasions of Czechoslovakia, Poland, and the Soviet Union. Along this line of thought, WWII in Europe started with Hitler and the Nazis invading Poland.

Of course, you could also argue that the Treaty of Versailles, the Great Depression, Soviet communism, and European anti-Semitism had all, in different ways, led to the possibility of WWII. Would Hitler have risen to power if the Germans were reconciled with a more forgiving conclusion to WWI? Would his promises to create jobs have excited the average citizen as much if the Great Depression had not exacerbated the German economy? Did the rise of Nazism benefit from the fear of the internationalization of communism? Did anti-Semitism persuade people to support or at least overlook the nastier aspects of Nazi ideology?

By the way, I'm new to Reddit. Is that enough in terms of providing sources? Or do I need to do some kind of MLA-type citation?

EDIT: Added a citation

Hitler, Adolf, and Michael Ford. Mein Kampf. [Camarillo, California]: Elite Minds, 2009