Catholic Encyclopedia: Persia
The history, religion, and civilization of Persia are offshoots from those of Media. Both Medes and Persians are Aryans; the Aryans who settled in the southern part of the Iranian plateau became known as Persians, while those of the mountain regions of the north-west were called Medes. The Medes were at first the leading nation, but towards the middle of the sixth century, B.C. the Persians became the dominant power, not only in Iran, but also in Western Asia.
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