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The Persian Empire .
The Persian Empire and a couple of hundred of the city-states of the Greek world.
The Delian League was charged with fighting the Persian Empire during the Greco-Persian Wars.
Zoroastrianism was the new religion that developed during Darius's rule of the Persian Empire. It was founded by the prophet Zoroaster (Zarathustra) and emphasized the worship of Ahura Mazda as the supreme deity and the belief in a cosmic struggle between good and evil.
Some of the Greek city-states joined with Persia, the southern cities opposed the Persian attempt to absorb them into the Persian empire.
The question is confused. The first king of the Persian empire was Cyrus. The king during the punitive expedition against Athens and Eretria in 490 BCE was Darius. The king during the invasion of Greece 480-479 BCE was Xerxes.
Pony express
The "Immortals" (sometimes called "Ten Thousand Immortals" or "Persian Immortals") was the name given by Herodotus to an elite force of soldiers who fought for the Achaemenid Empire. This force performed the dual roles of both Imperial Guard and standing army during the Persian Empire's expansion period and during the Greco-Persian Wars. The force consisted mainly of Persians but also included Medes and Elamites.
Conquest by another city, Athens' encouragement of democracies within it's empire, Athens being stripped of it's empire after it lost the Peloponesian War, absorption by the Persian Empire, liberation from the Persian Empire, and in the west, absorption by a tyrant in Sicily.
the Roman Empire and the Middle ages were run by priests who wore long black capes.
He was a religious leader who preached Ahura Mazda - the origins of today's Zoroastrians/Parsees.
During Darius the Great's reign at the end of the 6th Century BCE.