It’s a not uncommon theme in anime: some large imperialist/war nation or one associated with fire or occupying Japan.
It’s also worth noting that Japan had a history of imperialism and occupied a significant portion of the world around them not too long ago.
Japan has a pretty similar world view to us. I don’t know a lot about Japanese culture, but I think a lot of its similarities contribute to anime’s popularity in the US. We both have pretty rigid class structures, appreciate violence and capitalism and are enamored with technology.
I know that Avatar is American, perhaps I just wanted to air out a pet theory, however I think it’s good for us to explore some of these assumptions with art and stories.