Khosrow II ruled Iran from 590 to 628, conquered Syria, Palestine and Egypt from the Roman empire, and lost everything within four years of his high point.
Khosrow II ruled the Sasanian empire from 590 to 628 and took it to its greatest territorial extent since its foundation, holding Syria, Palestine, Egypt and much of Anatolia.
He is also the king who destroyed it. The war that produced those conquests exhausted Iran, and after Heraclius's counter-offensive his own nobles and his own son killed him.
Within twenty years of his death the Sasanian empire had ceased to exist.
He came to the throne in 590 after his father Hormizd IV was deposed and killed, and almost immediately lost it to a usurping general, Bahram Chobin.
He fled to Roman territory and appealed to the emperor Maurice, who restored him with Roman troops in 591 — an extraordinary intervention, paid for with territorial concessions in Armenia and Mesopotamia.
That debt shaped everything after. Khosrow owed his throne to Maurice personally, and when Maurice was murdered by Phocas in 602 he had both a genuine grievance and an irresistible opportunity.
He is harder to see than Heraclius, because the Persian sources for his reign are largely lost and what survives comes from his enemies or from much later Islamic-era compilations.
The consistent impressions are magnificence and detachment. His court at Dastagird and Ctesiphon was famously opulent, and he directed a twenty-six-year war largely through generals rather than commanding in person — the opposite of Heraclius's approach, and arguably the difference that decided it.
Later Persian literature turned him into a romantic figure: the Khosrow Parviz of the Shahnameh, and the lover of Shirin in a tradition that runs through Nizami. That is memory rather than evidence, and it should not be read back into the reign.
Phocas's murder of Maurice in 602 gave Khosrow his pretext, and he invaded claiming to avenge his patron. What began as a punitive war became a war of conquest.
His armies took Antioch, Damascus and then Jerusalem in 614, where the True Cross was captured and carried to Ctesiphon — an event that gave the whole conflict a religious charge it had not had before.
Egypt fell by 619, and in 626 a Persian army under Shahrbaraz stood on the Asian shore opposite Constantinople while the Avars assaulted the walls from Europe. It was the closest any power came to taking the city before 1204.
The coalition of 626 failed, and Heraclius was already deep in the Persian heartland striking at territory the armies in Egypt and Syria could not defend.
At Nineveh in December 627 the Persian field army was destroyed, and Heraclius marched on Dastagird, which Khosrow abandoned.
His nobles had had enough. In February 628 his son Kavad II deposed him and had him executed, made peace, and returned the conquests and the True Cross. Kavad died of plague within months, and the dynasty fell into a succession crisis it never resolved.
He came closer to destroying the eastern Roman empire than anyone until 1204, and the effort destroyed his own.
The two empires emerged from twenty-six years of war so weakened that neither could resist the Arab armies that appeared in the 630s. Iran fell entirely; the Roman empire lost its eastern half.
It is the clearest case in this archive of a war in which both sides lost, and the winner only more slowly.