I’m not convinced. If you want to argue that Japan-China was a local war, and Germany-France-UK-Poland was a local war, then how can you conclude that Germany-USSR was not a local war too? No other major power was involved in the combat in that theatre (partial credit to Italy); sure, the USSR was an Asian power, but (a) it didn’t actually fight in Asia between 1939 (Nomonhan) and 1945 (Manchuria); and (b) Britain was an Asian power too, so whatever war it was that started in 1939 already involved an Asian power.

Though I already rejected it in my post, December 1941 does have some merit for the reasons you say. It’s unquestionably the latest date by which the world war started — if it hadn’t started by then, it never would! Perhaps I was too hasty in dismissing this date, but I’ll still stick to my guns and say that June 1940 was actually when one of the wars spilled over onto another continent, and so it was already no longer just a European war. It could certainly be argued that Africa was just a sideshow (though it wasn’t for Britain and Italy), and the entry of Japan into the war forced some countries (US, Britain) to think and act globally. But my gut feeling is that the war was already well underway by December 1941, so maybe I’m just rationalising that feeling …

How about 22 June 1941? That’s when the European war involved an Asian power. It is when the scale of the war ratchets up dramatically. Even so one could argue it was still largely a European war.

How about Dec 1941? That’s when the European and Asian wars started involving the same participants and when the more-of-less final coalitions lined up as combatants. Didn’t AJP Taylor argue for these later dates, with the logic that this was when several very large regional conflicts coalesced into one world war?

Hobbes:

‘For WARRE, consisteth not in Battell only, or in the act if fighting, but in a tract of time, wherein the Will to contend by Battel is sufficiently known.’

Seems about right as a theoretical starting point.

Although I like Dan’s ideas as a way to get us to “think different” (especially the suggestion about the lives lost in effect due to war preparations), I think I’m still going to go with the traditional date. This is probably just my perspective; I’m more interested in popular and middle opinion than elite opinion. So Stanley might well have reason to think that in some important ways, the war had already begun by February, but if he had started going around saying “Put out that light! Don’t you know there’s a war on??” people would have looked at him very strangely indeed. Put another way, could Britain have won (or lost) Stanley’s war without 3 September, and the chance for actual fighting, having to happen first?

Similarly, while the Czech crisis gave people a real fright, and the occupation of Bohemia made war seem inevitable, the reactions of people on 3 September makes it clear that they thought something important had changed dramatically since 2 September (let alone 31 August). That was surely the biggest mental watershed of all.

Maybe the other way to look at it though, is as a series of gradations, from c. 1934 on, stopping off at the various pointed suggested by Dan, Chris and Scott. Then there’s no single (or simple) date for the start of war at all, but a slippery slope into war. I think this has virtues — it corresponds to the actual diplomatic process/situation better, for example. But if there was a slippery slope, at 3 September there’s a steeper grade, if not an actual vertical drop …

Respectful’s comment was interesting. It’s not so much a different way of approaching Jonathan’s point, as an Asian perspective on the second definition in my post — with the twist that the war is seen purely as a regional one, and not a world one at all. Is the idea of a world war just an Anglophone conceit, then? I have to wonder though, if museums in Beijing focus on Japan’s surrender while de-emphasising the atom bombs, how do they explain the surrender of Japan in the first place? Surely they don’t claim that China’s resistance to invasion, staunch though it may have been, was the cause? In other words, whatever national mythologies may say, I think WWII does need to be understood as a global war. (That’s not to deny that Britain has its own mythology of the war which can get in the way of understanding it as a whole!)

Finally, as to the First World War, I think that under my third definition it still counts as a world war — from the point when Turkey entered the war (October/November 1914). This made Asia and Africa potential (and eventually, actual) zones for major combat. Alternatively, one could consider the campaign in German East Africa to satisfy that condition, which would make it a world war from day one.

I think also, there are two separate questions in this post and the comments: (1) when did Britain’s war start? and (2) when did the war become a world one? An appropriate answer in the British context to (1) may not make much sense in the wider view.

Still, I’ve found the comments re: watersheds and inevitability interesting, though I can’t say that I agree with the various dates and events mentioned so far.

It seems to me that the Rubicon was only crossed, so to speak, on 23 August 1939 with the signing of the Molotov-Ribbentrop Pact which freed Hitler to go to war with Poland w/o having to fear Soviet intervention.

The view in south-east Asia (Malaysia, Burma, Indonesia) appears, from my limited reading, to be very similar to that in Taiwan.

As for ‘Munich as watershed’, surely if we’re talking about planning, we have to go back to 1934. If we’re talking about the spending of serious money, the watershed is 1937/38. If we’re talking about a consensus within the political class that war was now more or less inevitable, we have to go to the reaction to March 1938, notably the guarantees to Poland and Romania, which are pretty remarkable events, preceding as they did anything resembling negotiations or staff talks.

Why does this matter? I had a Taiwanese military officer friend tell me once they don’t consider the War of Resistance to be part of World War II, which somehow happened elsewhere. Likewise, how do you qualify World War I, whose theaters didn’t reach nearly as far across the globe as its successor; by your definition above (“If we are talking about a world war, then presumably it has to be fought on a world scale.“), does it count as a world war?