I’m not in the December 7th camp either. If I had to pick a date I would say September 1st 1939 since that set in motion a series of events that had, within days, the British and French at war with Germany. With most of the dominions following suit shortly after the war really became global at that point and hence a “world” war. But like I’ve said, there are some real good arguments for some of the other dates as well.
Btw, has anyone as of "today" seen a news report referring to the Second World War?
I have not, and wonder if that term was not used before December 7, 1941?
I'd even go back to 1936, when the Germans marched into the Rhineland, and the French didn't fire a shot.