Which factor played the most significant role in the start of world war ll

The most important cause of WW2 (as of WW1) was imperialism ... Japan started the war due to imperialist motives - to conquer China and Oceania for their resources and manpower. Neither the treaty of Versailles nor the failure of the League of Nations are causes for the Second Word war.

There are multiple factors that played an important role in the start of World War II. One of the most important ones is the Treaty of Versailles, that ended World War I. The Treaty had clauses that forced Germany to demilitarize its armed forces, it took territories that was once under German control, and it made the country paid for expensive war reparations to other countries, that put its economy in a depression ... These measures created, in the long term, a sense of revenge in German's society. These feeling of revenge is associated with the rise of Hitler to power because of his message to take Germany back to the place of power it once had.