What about the Sun Warriors though? Wouldn't that be Mayan or Aztec?

yeah they are mayan the art shown in the buildings is remeniscent of mayan culture

Then how can Fire nation be Japanese?

Air Nation is Tibetan Monks and the water tribes are Inuit.

Though china was invaded in ww2 by japan, if you were trying to link that to thw show, it would be incorrect.

Avatar was set probably . . . mid 1800's or so. Seeing as how Legend of Korra takes place in about early 1900's and Avatar takes place 80 or so years prior.

Water nation is korea

The war has caused the north to lose contact with the south dividing the two and causing cultural differences.

Water tribes are based off of the Inuit.

*water tribes
*air nomads

They are, like water bending uses tai-chi. He's saying that the actual nations are based upon real world civilizations. Though the martial arts thing isnt totally adamant, like toph uses something different from other earth benders, and pro-bending is based on boxing

Toph uses Southern Mantis Style and firebenders use Northern Shaolin.