Pick a date, any date
In a comment to an earlier post of mine at Airminded, Jonathan Dresner quite legitimately took exception to my use of the term 'interwar' to refer to the period 1919-1939:
From an Asian history perspective, the Japanese use of chemical weapons in China isn’t really “interwar,” as major combat operations began in late ‘37 (leading to the Nanjing Massacre, etc.) and ran continuously through ‘45.
While Jonathan is conveniently distracted, I thought I'd address the issue he raised -- essentially that of when did the Second World War start? Of course, this is a hoary old question, and the answer usually depends on where you're from. Australia's war started on 3 September 1939, the same date that Britain, France and New Zealand declared war on Germany. So we were in it from the start. Well, the start, bar the two days during which Poland was fighting alone. Or possibly the start, bar the two and a bit years since the Marco Polo Bridge Incident, as Jonathan suggests. (I hope we can all agree that the United States was too late to the party to have much of a say in when it really started.)
I have three (count 'em, three) responses to this. The first is an objective one. Standing from outside the Universe (as one does), looking at the war as a single event in space-time, it's clear that Jonathan is right. There's no question that China and Japan were fighting on opposite sides in the war; they were the first of the participants to start fighting; they started fighting in 1937; therefore the war started in 1937. 7 July 1937, to be precise.
The second is a subjective one. As an historian of Britain, and one who is largely concerned with the ways in which the next war was anticipated, it is more useful for me to take the point of view of the British people themselves. And very few of them thought that the Sino-Japanese War was the start of the next war, or even a prelude to it. It certainly showed Japan to be an aggressive, expansionist power, which one day might clash with the British Empire. And it also confirmed some ideas about the brutality of modern warfare, and added to the volatile atmosphere of the time. But it was essentially seen as 'a quarrel in a far-away country between people of whom we know nothing', as Chamberlain was soon to say about somewhere else. It was a war alright, but not Britain's. (If there was a prelude which threatened to pull Britain into a general war, it was the Spanish Civil War, but this possibility faded over time and it never did join up with the larger, later war.) So from this (my) point of view, 3 September 1939 is the start of the war, as that's when Britons thought their war started, and this is the date I will use in practice.
The third and final response is, ummm, a geographical one? If we are talking about a world war, then presumably it has to be fought on a world scale. That rules out 1937, and it rules out 1939 too (because it was not yet joined with the fighting in China, and leaving aside skirmishes like the Battle of the River Plate). As a rule of thumb, we could perhaps say that there needs to be the possibility of intensive ground combat on at least two continents for it to be considered a world war. For the Second World War, this would be when Italy declared war on Britain and France. That opened up Africa as a potential combat zone, in both the north and the east of the continent. And so on this basis, 10 June 1940 was the start of the Second World War.
So, there's three dates: 7 July 1937, 3 September 1939, and 10 June 1940. I'll stick with the middle one as it's most useful to me, and I make no apologies for that. But by the same token, others can and will have different dates in mind. What would you pick, and why?