The domino effect of the network of alliances amongst European countries, is what turned Hitler's territorial ambition in E. Europe into a world war, the same way the "Great War" started. That, and America's commercial and possibly humanitarian interest in not letting the Far East be conquered by Japanese millitarists. The Roosevelt administration, btw, cannot be criticized too harshly, it did everything it COULD to help Britian's cause and bedevil Japan's ambitions, short of blatantly breaking neutrality. One must remember that Roosevelt had BARELY defeated Wilkie, a candidate whose only appeal was swearing to keep the U.S. out of war, by making the same promise. It took a Pearl Harbor to make America interventionist.
So essentially, the war happened because of German and Japanese aggression. Unfortunately for humanitarian idealists, the E. Europe we just saved from Hitler was swallowed up by Stalin with barely a peep from the West. (Returning Polish and Czech pilots who had fought in the BoB and beyond found themselves thrown in gulags, often enough.) China was allowed to become Maoist with little more effort, with attendant body counts that make Nanking look amateurish. Then the Most Powerful Nation on Earth belatedly decides to half-heartedly fight for the right of small nations to be free in two of the most awful backwaters of East Asia, winning neither conflict.
This stuff about Hitler and his mad scientists taking over the world with nuclear-armed jet-powered flying wings and other "Sky Captain" storylines is a revisionist&History Channel fib; Great dramatic and speculative fare, but not something most Americans at the time had in their minds as a reason for war.
I'm as patriotic a U.S. citizen as anybody (some would call me jingoistic), but in scoring WWII, I got to say, Fascism struck out, Western Democracy got beaned, and Communism scored, or was rather given, at least two homeruns.