Complex sentence: By the 6th century BC, after having allied with the Babylonians to defeat the Neo-Assyrian Empire, the Medes were able to establish their own empire, the largest of its day, lasting for about sixty years.
Split into simpler sentences: By the 6th century BC, after having allied with the Babylonians to defeat the Neo-Assyrian Empire, the Medes were able to establish their own empire. It was the largest of its day, lasting for about sixty years.