There are multiple factors that contributed to the beginning of WW2.
The first one is the Treaty of Versailles. After WW1, all the blame for the war was placed on Germany and they were forced to pay massive reparations. This proved ruinous to their economy, and the struggled very much during the 20's. This allowed for a charismatic leader (Hitler) to take power by promising the people that he would be able to fix their economy and restore Germany back to its previous power. He decided that the best thing for Germany would be to get more land, and he decided to do this by attacking surrounding countries.
Another thing that led to war was Japanese imperialism. Japan was a small island nation with few resources and a large population. They thought that the best way to provide for their people was by invading other countries and taking their resources. This is why they attacked Manchuria and China in 1937.
Another reason why WW2 started is because of the failure of the League of Nations. The League of Nations was formed after WW1 to prevent such a war from happening again. However, when Japan invaded Manchuria and Hitler annexed the Sudetenland, the League of Nations did nothing. They feared that if they got involved, they would start a war. They thought that compromising with Germany and Japan, rather than condemning them, would be a better way to avoid war. The League of Nations made a deal with Hitler that he could annex the Sudetenland as long as he did not attack anywhere else. Hitler agreed, but he went back on his plans. This was a failure on the part of the League of Nations. If they wanted to avoid war, they shouldn't have tried to compromise with a dictator.
