I'm surprised such a superstitious reply is so highly-upvoted. There's no "WW3" any more than there is time travel or blue smirfs. It's a hypothetical, but you're talking about it like it's an inevitability. That's just not logically-sound thinking.
It was only when one stood back to regard the whole picture that it became clear that something larger was happening.
OP is making the same point.
https://en.wikipedia.org/wiki/Declarations_of_war_during_Wor...
Of course it took longer for it to blow up into a truly global war (Pearl Harbor etc), but a conflagration across Europe is hardly a "small regional war".
Once Hitler invaded France the "phoney war" turned into a real war. [1]