The start of the War is generally held to be 1 September 1939, beginning with the German Invasion of Poland. Immediate trigger for the spread of World War II came with Britain and France declaring war on Germany. “On September 1, 1939 Germany invades Poland causing the World War II.” (John Allen) pg.16
Hitler wanted to be a ruler and have World Power. They invaded Poland, first, so they couldn’t get better access to other countries. They tried to take over everything and they also killed Jews, Hitler hated the Jews because and that still remains a question. Maybe the Religious conflict, Jewish economic power, Biological differences and perhaps a conspiracies. Britain and France united to go against Hitler.