The Persians

In my review of ancient history, I've progressed from the Babylonians to the Persians, and from Cyrus through the lesser kings to Darius.  I'm working my way toward Xerxes so I can have some good background on the book of Esther.  This is the book I read:

Persian Fire: The First World Empire and the Battle for the West

What are the differences so far between the Babylonians and the Persians?  The Persians let all the conquered peoples stay in their own areas, retaining their own language, culture, and religion.  To some of those they conquered, this strategy made them look weak, so there were always uprisings.  In addition, the Persian language never became the lingua franca of the areas they conquered and their holdings were vast and extended from modern day Turkey to India and through northern Africa to Ethiopia. 

Why are the Medes and Persians tied together?  Why do we call it the Media-Persian Empire?  The story goes (there is historical support for this story) that the King of the Medes married his daughters off to the Persian King.  But he had been told by his advisors that a grandson would murder him.  So the King of the Medes had his chief lieutenant steal the child and kill it.  However, the chief lieutenant decided to hide the child instead, and then inform the Persians where the child could be found.  That child was Cyrus.  The lieutenant remained a double agent and when Cyrus became king, he assisted Cyrus in killing Cyrus' grandfather and uniting the Medes and the Persians.  He became Cyrus' lieutenant and the famous Median cavalry joined the Persian armies and navy, and together the Medes and Persians became a world power.   Sometimes subterfuge backfires, but in this case it served the Persians well.