Persia is a dry land of nothing that was formerly part of the Squadala Empire, until the Persians rebelled. It is the primary enemy of Sparta and the 300. It was also the most worthless part of the Squadala Empire. We don't even know why Squadala Man bothered to conquer this lifeless desert. All we know is that one day, he ran in without any pants and screamed "SQUADALA!" and the land was his.
Persia is only known because Leonidas wouldn't stop crying about it during the Spartan War. He screamed "THIS IS SPARTA!" and the Persian guys screamed "THIS IS PERSIA!" and then Leonidas kicked them because it was in fact Sparta. Persia, besides its involvement in the Spartan War, is also well known for its production of funny hats.