For one thing, the story seems to take place in 1914 and 1915, I saw the year on Hughes's grave, which was the year of WWI
As for the other thing, we got Führer Bradley, which does seem to maybe be some WWII reference, and not just because he is an evil Führer
The homunculuses think they're better than everyone else, just like how the Nazis thought they, as Aryans, were better than everyone else
As for Xing, Xerxes, and Drachma, they seem to be more based on older civilizations (Since the last dynasty of China died in 1911)
Xing is obviously some Chinese Dynasty, which has always had the problem of having their emperor die, cause then all their sons would fight for the throne, which is referenced with the whole thing with Ling Yao trying to find immortality
Xerxes seems to be based on Ancient Greece mainly due to the scientific advancements they had, their architecture, the clothing, and how alchemy came from there
As for Drachma, we didn't see much of it, but I'd guess they're either based on Vikings or the Swedish Empire that existed in the 17th century
and of course, I'd say Amestris is obviously based on Germany
and I'll just skip the Ishvalans