Although not totally correct from timeline perspective, the most notable event considered by many as the beginning of WW2 is Battle of Weterplatte. The invasion of Poland had began in multiple places between 4:30 am and 5:00 am on September 1st in 1939.
Battle of Westerplatte ... interesting piece of history:
https://en.m.wikipedia.org/wiki/Battle_of_Westerplatte