What event sparked the start of ww2

Germany invaded Poland and Britain and France responded by declaring war (WW2)

There are many different theories, but the first invasion was when Germany started an unprovoked attack on Poland. Following Germany's first invasion, GB and France declared war on German. The US entered after Japan bombed peril harbor.

In my opinion, I think Japan was the underdog and wanted some action as well as power. Little did they know, so were the pilots that were flying over Nagasaki and Hiroshima.