Some causes that led to the start of WWII:
1) War reparation: Seen as unfair in today's society, the Allies decided to put most of the war costs on Germany (even though they did not actually want the war to start, and even warned Austrio-Hungary against it.)
2) Great Depression: Because of the above, the Kaiser became unpopular, and had to step down. Large amounts of debt meant more bank-note printing, which led to the decrease in value of German currency.
3) Rise of Nazi Party: Combined with a downturn of the economy, and then the Great Depression on top of that, the then small Nazi Party was able to grow in size, and was able to coup de'tat the government, replacing the conventional government with Hitler, which rose from being the leader to chancellor, and then the supreme leader.
4) Greed: Countries such as Italy and Japan did not believe that they got what they deserved, and so turned joined the Axis Power in hope of being able to conquer more lands, take resources, and effectively create a world with their ideology inside.
If you want to know more, you can look at my previous answer (/11249535).
hope this helps
