"Fullmetal Alchemist"'s use of fantasy elements is interesting to me because of the way it... reduces... certain horrors to these unflinchingly direct basics. (Warning for spoilers and unorganized overview discussion of canon-typical violence.)
I was thinking a similar thing this morning, about how the story uses the extremely simple and geometric political organization of the state to foreshadow its nature and intention as a construct for centralizing power into a single person.
Because until the reveal, the roundness of Amestris and its organization around a centrally located capital named 'Central' and regional capitals called after the points of the compass, all linked by astonishingly direct rail lines, looks like it's just worldbuilding done in a shorthand style, so the audience finds the layout of the country and its implicit power systems immediately legible, without any need to spend exposition on it.
Which is also a function it performs!
But in the real world, arrangements like this always reflect a centralizing power structure rearranging space, infrastructure, and so forth to make its territory more legible and controllable by the central power-holders.
19th century France even as it rotated through different governments is heavily defined by its efforts in this direction; ludicrously expensive railroads were build radiating out from Paris along strict geometric lines, rather than accommodating themselves to the terrain and existing habitation patterns like almost everywhere else in the world that put in trains. (Even in the American West and Siberia, straightness wasn't prioritized over practicality the way it was on the line from Paris to Strassburg.)
Arranging civic affairs in this abstract way works, in theory and usually in practice, to allow government immediate and easy access to its territory and the ability to extract resources from it, such as taxes, conscripts, and so forth.
So in revealing that the country is Shaped Like That because the central authority conquered exactly the amount of space it needed to later convert the public into a consumable resource on the site of the capital, the fantastic consumption of Amestris' citizens by its shadow government is prefigured by and works more clearly as a reflection of the realistic exploitation of centralized power in real societies.
Thus the immortal monster that wants to open a hole in the world and become all powerful through massive human sacrifice reinforces, rather than countering or undermining, the narrative elements that are about how fascism works and why that's bad.
