The cause of WW2 is Nazi Germany.
Under the Weimar Constitution, Hitler formed the Nazi Party and led Germany again to rearmament.
And Nazi Germany invaded neighboring countries like Poland, France, and the Soviet Union... This resulted in the extension of war across Europe.
Furthermore, the development of the atomic bomb, which promptly progressed, was caused by the fact that the United States was so concerned about the development of the situation in Europe caused by Nazi Germany.