The Persian Empire, in antiquity, was the largest geographical empire in its time, at its height stretching from India (at the Ganges and Mula, respectively) all the way into southern Europe and down to Egypt, primarily under Cyrus the Great. It had not known defeat until it was turned back in its efforts to conquer Ancient Greece. It had two capitals: Susa and Persepolis.
The Persian Empire came into being in 539 B.C. with the defeat of the Babylonian Empire and continued under other famous rulers such as Darius and Xerxes until it was conquered and largely absorbed by Alexander the Great in 330 B.C. The Persian Empire and several of its rulers are discussed in the Bible in the books of Daniel and Esther.
A later Persian Empire re-emerged in 226 A.D. and outlived the Western Roman Empire. After a devastating war with the Byzantine Empire that left both sides severely drained, Islam came on the scene from what is now Saudi Arabia and completely conquered the Persian Empire in a quick campaign that ended in 651 A.D.