I've been listening to Patrick Wyman's excellent podcast series, Tides of History, and recently heard the episodes about the Hundred Years War. I actually have a lot of questions about the war but the biggest one is, why is the Hundred Years War treated as one conflict but the World Wars aren't?
From what little I know about the Hundred Years War, it was 116 years of conflict broken up by periods of peace that sometimes lasted more than a decade. Not only that, the war "ended" in 1453, but the English invaded France again in 1475; however, this subsequent invasion is not considered part of the Hundred Years War. Also, during the 116-year time that the war raged on, several distinct political eras came and went in both England and France, which is not unlike what took place in the major powers that fought in the World Wars.
On the other hand, World War I and World War II are treated as two distinct conflicts by most historians (I'm aware that some historians look at the World Wars as a single conflict, but that view is far from mainstream). This is despite the fact that there are clear, obvious links between the World Wars, cause-and-effect relationships all over the place, and so on. Also, while the period of peace between the World Wars was longer than some of the periods of peace during the Hundred Years War, the long period of peace between 1389 and 1415 was a full five years longer than the peace between 1918 and 1939. I acknowledge that there was internal factional conflict going on in France during that time, but the same can be said about Germany during the peace between the World Wars.
The one glaring shortcoming in my argument that I'm able to identify right away is that Japan's role in World War II does not fit into the "World Wars as a single conflict" paradigm. However, some already consider the Pacific War as a distinct conflict, and I think that's reasonable: Japan was already at war for several years by the time Hitler invaded Poland, and while Germany and Japan did cooperate in some ways, they did not really support each other militarily in the way that the Allies supported each other.
I'm very curious to know what you all think about this. Thanks in advance for what will surely be a fascinating discussion!
This is kind of a tricky question, but I'll do my best because there's two ways to approach it. I would essentially boil down my answer to two points, which I'll expand upon a bit:
I'm mostly going to talk more about the first topic, because I don't think there's much more I could say on the second. I'm not enough of an expert on nineteenth century historiography to really comment on how The Hundred Years War formed as a concept, nor am I specialist in WWI or WWII, so I've kind of reached the limit of what I can say on that. As to the first topic, though, I can say a little more.
Most historians currently writing on the Hundred Years War acknowledge that it is essentially a modern construction. There are two avenues of deconstruction for the HYW, one is to split it up into multiple sub-conflicts, while the other is to see it as part of an even longer string of Anglo-French conflicts stretching back to the early days after the Norman Conquest of England and continuing on until at least the Tudors.
Both of these ideas have their merits, but generally the idea of breaking it down into smaller conflicts is more popular than viewing it as an extended single conflict. This isn't that surprising when you consider that studying smaller pieces of history in greater detail is generally preferred in academic history to attempting massive sweeping histories of huge subjects. A book on Edwardian Warfare 1337-1360 is much easier to manage than Anglo-French Warfare 1066-1600, y'know?
From this perspective, it can be tempting to think that the concept of the HYW sticks around basically because it's a successful "brand", if you will. People know what the HYW is because it exists in popular culture and imagination, and historians keep using it because it's a useful shorthand for "War Between England and France in the late 14th and early 15th Centuries".
This is definitely a big part of why it has stuck around so long, but some historians have argued that it is still a useful concept. There are some aspects of the HYW that distinguish it from it's preceding and following conflicts. The big one is the English monarch's claim to the French throne. Before Edward III, Kings of England may have fought the French king, but they didn't claim that their purpose was to depose him and take his place. Now, this claim did continue on into the reign of Henry VIII, but by then it was largely meaningless. While Henry V - and to some extent Henry VI - could be seen as plausibly able to make a practical argument that they ruled France, Henry VIII controlled one city and his wars in France achieved very little.
So, to some extent there are key ideas and themes that unite the multiple conflicts of the Hundred Years War into a coherent whole that helps to define it as a period - if not as a single conflict. One could compare it to the idea of the Age of Crusades - while each Crusade is its own military conflict, the first 7 or so all have unifying themes that link them together into a broader concept that historians use to talk about that aspect of that period of time (I hope that makes sense, I'm not convinced it does..)
If you're interested in learning more about the Hundred Years War I highly recommend David Green's The Hundred Years War: A People's History, it's a great introduction to both the conflict and that time period in England and France.
/u/Valkine has done a very good job of covering the Hundred Years War portion of your question. However, there is also the part of your question that asks (roughly) "then why aren't the World Wars considered one conflict".
I was going to write up an answer to that, but it turns out I wrote an answer on this topic two years ago which captures my thinking on the topic. Be sure to click through the links in that answer to the posts by /u/restricteddata where he expounds on ideas of historical categorization and narrative.
I'd also recommend checking out this answer from /u/Superplaner that explains strengths and weaknesses of the "Second Thirty Years War" and "European Civil War" theories, which both see the two wars as a single (European) conflict. That thread even includes a discussion of historiography of the Hundred Years War!