Why did the World War 2 start?

Answer 1

The Nazi Germans wanted to have more land for their people, so Hitler started conquering countries. And he attacked Poland, leading to France and Britain declaring war on Germany for attacking their ally.

Answer 2

There are many different causes for WW II, but the most important cause was lingering resentment about WW I, and the Treaty of Versailles by which that war was formally concluded, which the Germans considered to be extremely unfair toward themselves.