Melissa from Tennessee asks, “What is the standard among (medieval) military historians for naming the Hundred Years War? Should it have an apostrophe or not?”
Great question, Melissa! I always leave it off. I saw one historian exclude the apostrophe and I had followed suit ever since. However, I understand why you would question the standard, because there is no standard.
Among today’s prominent medieval military historians, Kelly DeVries (sometimes) and Anne Curry (always) use an apostrophe, but Jonathan Sumption and Clifford J. Rogers do not.
The Oxford Encyclopedia of Medieval Warfare and Military Technology uses it, but the Journal of Medieval Military History does not. Funny enough, three of the historians above have contributed to or edited both of these works.
Then there is Wikipedia, which uses it.
An Ngram shows that English writers preferred an apostrophe for the past two centuries, but by 2000, you could find just as many instances without an apostrophe as you could with one.
If we move onto other wars with the number of years in the title, the situation becomes more muddled. For example, the Seven Years War enjoyed its peak in apostrophe use in the late nineteenth-century, but by 1980, with our without would do just fine.
The Thirty Years War experienced the same.
The best advice I can give you on whether to use an apostrophe in naming these wars is to be consistent.
As for me, I prefer leaving the apostrophe off for simplicity.