The cause of WW2 is Nazi Germany.
Under the Weimar Constitution, Hitler formed the Nazi Party and led Germany again to rearmament, and caused social change in the country, with Hitler turning economic worries into ethnic hatred.
Nazi Germany invaded neighbouring countries like Poland, France, and the Soviet Union... This resulted in the extension of war across Europe, a war in which German Forces carried out hundreds of massacres that constitute war crimes. Not to mention committing one of the worst organised mass murders in history, killing six million Jews, Romani, Serbs, Poles, criminals, prisoners, and disabled people in the name of Eugenics.
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.