From Ohio History Central
World War II formally began in September 1939, with Germany's invasion of Poland, although military aggression had taken place between various countries of the world for several years before that date. Germany had been conquering its central European neighbors since the mid 1930s, while Italy had conquered vast amount of territory in North Africa. Japan had also used military force to expand into Asia beginning in the early 1930s.
There are numerous reasons for why World War II began. Among the more important ones ranks a desire for Germany to reclaim land seized from it following the German defeat in World War I. Germany, Italy, and Japan also helped their people cope with the Great Depression by building up their respective militaries. Jobs opened in defense plants, as these nations expanded their militaries. Other men found jobs in the armed forces of these nations. As these nations became militarily stronger, nationalism and a general desire to be the most powerful country on the earth prompted them to expand.