The Medo-Persian empire was composed of the Medes in present-day Iraq and the Persians in present-day Iran. The Medes were the dominant ethnic group initially with the Persians becoming stronger. Daniel 7 illustrates this with the bear leaning on one side first and then the other.
Dan. 2:5 And behold another beast, a second, like to a bear, and it raised up itself on one side, and [it had] three ribs in the mouth of it between the teeth of it: and they said thus unto it, Arise, devour much flesh.