The cause of ww2 can’t be really quantified in a Reddit post but I’ll try and make it brief. WW2 is basically caused by the German hate of the treaty of Versailles. Germany had to pay astronomical debts to countries, money it did not have, had to demilitarize the Rhineland, the region that borders France, and overall lose a lot of world privileges. This caused a lot of hate in the country and the Nazi party gained a lot of following. Keep in mind however, that throughout the entire war, the Nazi party was a minority, a loud one, but still. Anyways, crippling debt, anger towards the allies and a need for what Hitler called « lebensraum » or living space caused Germany to want to gain more territory and also reunite the « German people ». This caused him to gain Austria through its annex, as well as the Sudetenland, the boarder state of Czechoslovakia. Then Poland, split in half by Germany and the USSR after a deal. Germany before this also de-militarized the Rhineland. He tried to take countries like the Netherlands, and when he tired Belgium, England declared war. All of this is to say that this was on the European Front. On the Asian Front, Japan was taking a lot of land from Korea and China for the same reasons. They needed space for its own people, as they had a population boom in the last years.

WW2 is interestingly enough, a war over resources as the only two viable ways of making money as a country is either producing or taking land. Some countries needed more land to farm and make food, along with some psychotic dictators, and too strong nationalism. Germany Japan and Italy all chose to take land until someone said that was enough.

I hope this clears it a little more for you.