What was the main reason for Hitler to start World War II?
One of the main reasons was simply an expansion to the borders of what he viewed as Germany, to take back the Rhineland and annexing Austria. After that it was to continue to further the goals of the Nazi party and the Nazi ideal, to expand the borders of what Hitler considered to be a righteous republic while also helping to rid the world of the people Hitler considered to be standing in the way of progress and civilization.
Some might also argue that it was simply a natural consequence of the poorly drawn up agreement ending WWI. There is also the argument that he had built up the military to such an extent that it simply had to be used.