Recall how WWII almost never began:
After an extremely hostile reception in the British parliament to his last minute attempts to avoid fulfilling his promise to go to war with whoever invaded Poland, Neville Chamberlain and his cabinet met in a mood of grim determination , abetted by an ominous thunderstorm from Mother Nature.
They finally voted, late on that evening of September 2nd 1939, to send Hitler a blunt ultimatum --- one with a very short response time, after which they would immediately declare war on Germany.
This turned a local war between a Great Power and a small power, one not greatly different from Hitler's smoothly successful earlier invasion of the rump of Czechoslovakia on March 15th 1939, into a full blown global war.
The UK actually declared war on September 3rd, but almost all historians agree that it was this cabinet decision the evening earlier that really launched WWII.
WWII, they say, certainly didn't begin with the Japanese invasion of China in 1931, or the invasion of Eithopia by Italy in 1935.
A world war needs formal war declarations between at least two Great Powers to truly make it so.
A formal war declaration between two fairly equally sized Great Powers ensures that the resulting conflict would be long, fiercely fought and a global fight.
So they see WWII as growing by a few key dates :
In 1939 the French empire joins the British empire in declaring war on the German empire.
In 1940, the Italian empire declares war on the British and French empires.
In 1941, the German empire declares war on the Russian and American empires, and Japan declares war on the American, British and French empires.
The Russian empire declares war on the Japanese empire in the dying moments of WWII, in August 1945.
But I will argue that there was in fact only one key date : September 2nd 1939.
If Great Powers don't seek a compromise peace after formally going to war with another Great Power - and WWI and WWII certainly suggests this to be the case - then WWII began with this formal war declaration of the UK to Germany which had to end with the defeat of one or the other side.
But could we predict which one would win on September 2nd 1939 ?
I say yes : the UK.
In 1939, the UK's global strength was not really its Empire.
Instead it was really anchored by several - distantly remote from Western Europe - clusters of British-oriented but nominally independent Dominions.
In the White Dominions, most of the population in control were fairly recent immigrants from the UK : think of them as the UK abroad rather than as reluctant colonies ever willing to change sides to go with the new winner.
India, for example, might have abandoned Britain if she was really on her uppers.
The white Dominions really being (at least in 1939) extensions of Britain itself, would not give up so readily.
So Germany would first have to defeat all of the British Isles and Eire --- perhaps a fairly do-able task in 1940.
But then they would soon have to take on all of Canada and Newfoundland, filled with fleeing diehards from the UK, if they wanted to feel permanently secure.
(And probably America too, if at some point it seemed Germany might defeat Canada.)
And then South Africa and the nearby White dominated British African colonies.
And then Australia and New Zealand and their mandate territories.
It was as if Napoleon thought he could defeat four (4) different Russias in succession.
Churchill or Britons of his ilk (and there were many of them) if they did lose the UK to Germany, would not just give up.
Instead they would fight a slow delaying rear guard action from Dominion to Dominion confident that Hitler's racist policies would wear out his welcome fairly quickly, no matter how much of the world he held by force of German arms.
But if Hitler had attacked only the French empire and Britain for some reason had remained neutral, would the French overseas territories have fought on and on after the Fall of France?
Not in 1940 , they won't have had.
But those three Dominion clusters, each the size of Western Europe, were the anchors that would have ensured that some British-led coalition would have ultimately defeated Hitler regardless of how luck and his decisions had worked out.
He lost his war the day it began : it just took six years to make it official ....