I absolutely love Attack on Titan and it's one of the best anime's I have ever seen. Even the clown show ending from the manga has been fixed to an great extent in the anime. The anime has so many excellent moments which are some of the best written scenes in anime.
The fanbase is insufferable though. It's either full of "Eren is literally me" mfs or glazers with no sense of reality and I'm pissed at liking the same thing as them.
It's supposed to be a dystopia in a sense. Paradis is a regressive island stuck a hundred years behind a already backwards world. It's pretty much set in what would be the 19th century in the real world. I don't think it's really about Nationalism in the first three seasons.