re: WWII dates. Germany and Poland starting fighting on Sept. 1. Britain and France didn’t declare war for several days. The British dominions took a few more days (except for Ireland which declared neutrality). The United States didn’t declare war until the end of 1941. The Soviet Union entered the war on the side of the Germans in September 1939, and switched sides two years later when Germany attacked them. The Japanese and Chinese had been shooting it out for some time, and Ethiopia had been at war with Italy since 1936. Arguably World War II ended around 1991 when the Allied occupation of Germany finally came to an end.
World War I began in a more organized way, reflecting the stable and structured world of the “proud tower” (as one historian called it) that the war undermined.