What was the main cause of world war 2?

The main cause of WW2 was German (Italian, Japanese) aggresive foreign "policy".

Germany was unsattisfied with the state of affairs after WW1. Poverty, loss of colonies, and humiliation led to wide support for radical movements. As a result Hitler seized power.

His solution for Germany's problems was to cleanse Germany of inner enemies (like Jews), restore pride to German people by creating strong army (which violated the treat of Versailles), and giving more "life space" for Germans (so that they wont suffer from poverty).

In order to create a strong German, Hitler annexed Austria. Next target was Czechoslovakia. Since there was no reaction to those aggressions from western powers, Hitler demanded from Poland incorporating Danzig to Germany and creation of highway connecting German mainland with Eastern Prussia. Poland refused and so Hitler attacked on 1st September 1939 ...

As months and years passed more and more countries joined the war and it became truly international.

It's a complex question, I hope i have shed some light on it.

The Germans rounding up the Jews actually wasn't a cause at all, in fact most countries ignored that until almost the end of the war. WW2 started when Hitler invaded the Rhinelands and took over Austria, as well as him building up his army, both of which directly violated the Treaty of Versailles which had ended WW1