Chosroes II was brought to power whilst his father yet lived. In a manner not overflowing with filial regard he then had his father executed. However, he was subsequently chased from the throne by the rebellion of a talented general (it was his own fault, as he abused the chap and made him dress as a woman in front of the troops), and fled to the old enemy of Byzantium (then ruled over by Maurice).
In return for some regained territory, the Byzantines agreed to help Chosroes get his empire back. The aid of the Byzantine Empire enabled him to defeat his enemies in battle (592 AD) and he once more ruled over Persia. A period of peace naturally followed, as he was understandably grateful to the Byzantines.
Maurice was overthrown by the vicious thug Phocas in 602 AD, and as the emperor’s life expired so did the gratitude Chosroes felt towards Byzantium. The armies of Persia overran Byzantine territory and reclaimed and plundered a good deal of it. The important province of Egypt fell to the Persian armies, and Byzantium, led by a cruel and divisive man and also suffering invasions from barbarians in Europe, could do little to stop Persia’s purple patch.
Phocas’ reign was not very long, and he was slain and succeeded by the talented Heraclius, who sailed from the Exarchate of Carthage (which had never acknowledged Phocas’ authority) to avenge Maurice’s death. Under the leadership of Heraclius, who reigned for over three decades, the Byzantine armies took the field against Persia, and scored notable successes.
Chosroes II fled from Heraclius, and was murdered in his own palace. As events unfolded, he turned out to be the last powerful Persian ruler, as the empire was torn apart by dissension and then fell to the zealous victories of the newly born religion of Islam.
In a period of a few decades Chosroes had gone from being ruler of Persia, to a refugee begging for help, then became a grateful ally to Maurice, a threat to Byzantium and then the victim of Heraclius’ military talents. Persia had waxed brightly before waning rapidly, and the empire that had been founded by Cyrus, destroyed by Alexander and reborn from the ashes of Parthia was no more.