Note that Iranian and Sassanid will be used interchangeably, and Roman will replace Byzantine or Byzantium.
The year is 627. For decades, the Roman Empire and Sassanid Empire were locked in one of the bloodiest and the last wars of antiquity. In this year, fortune favored the Roman cause as Emperor Heraclius prepared to march on the Sassanid Capital of Ctesiphon. However, this was a new development, and initially, it would have seemed that total victory was at hand for the Sassanid Emperor and its Shah, Khosrow II. Nearly all of the Eastern holdings of Rome had been taken, Syria Palestina, Anatolia, Egypt, the Caucasus, until all that stood between the Empire and total destruction were the wall of Constantinople, then under siege on both sides by the Slavs, Avars, and the Sassanids.
The situation was so dire for Rome that it got to the point where Heraclius, newly crowned, offered to make Rome an Iranian client state. Seemingly having inherited little of the original’s wisdom, Khosrow II refused, believing total victory to be at hand. This was not to be, however. Urged by the Patriarch of Constantinople to stay and fight, Heraclius dedicated all funds that could be diverted to restoring the Roman army. By 624 he launched a counter-offensive, sweeping through Sassanid-occupied Anatolia and Caucasus. The last dirge of the Sassanid attempt to capitulate Rome would be in one last attack on Constantinople in 626, which ended again in failure.
With this opportunity, Heraclius took the initiative, but he would not waste men and resources retaking the Middle East and Egypt; his goal was a direct attack into the Sassanid Imperial core in Lower Mesopotamia, a straight shot at Ctesiphon, but between his army and the Iranian capital was the last general at Khosrow’s disposal, Rhahzadh the Armenian. While the rest of Khosrow’s generals were either killed or dispersed, Rhahzadh, a rather rugged and heroic figure, was given command of an emergency force of 12,000 men. He shadowed, waiting for reinforcements and an ideal opportunity to engage.
After the reconquest of Anatolia, Heraclius was indeed marching southward. Still, with
this new Iranian army tailing him, he would choose a more advantageous position to engage, open enough to maneuver but close enough to home territory to retreat. In 627, near Nineveh, the Roman Empire and the Iranian Empire would meet in one final confrontation. In the early morning, both armies drew up. This battle had to be decisive.
Rhahzadh drew the proverbial line, behind him were open cities and the capital of his empire; the Romans had to be stopped. Heraclius likewise understood winning could secure Rome’s recovery. The fate of both Empires lay in the outcome. Both sides drew their armies to the field for battle. Rhahzadh, despite being outnumbered, seized the offensive and attacked the Roman line, his infantry in the center and the elite Sassanid cavalry, the Asbaran, on the wings. Heraclius was prepared. His forces feigned retreat, baiting the aggressive Sassanid forces into a disorganized advance. As they advanced, the Iranian formations lost cohesion, giving Heraclius his chance. The Roman forces reconstituted and engaged the Sassanids, who began to buckle under this new pressure.
At this critical moment, some sources claim that Rhahzadh challenged Heraclius to single battle. As the Iranian forces began to waver, Rhahzadh realized that the only path toward victory would be to engage and kill Heraclius in single combat, undoing the morale of his troops. Amidst the carnage of the battle, the great duel commenced; however, Heraclius found his opening early and beheaded Rhahzadh in a single swing. Again, I stress that if this account is accurate, then one must commend Rhahzadh for his bravery. Upon the death of the Iranian commander, his troops finally broke, disengaging and retreating. Despite the Roman Emperor’s victory, his march south ceased with an attack on the fortified Ctesiphon becoming untenable, his own forces unable to complete such.
In the aftermath, Khosrow II was killed in a coup d’etat, while his empire was subject to civil wars by Imperial claimants before being ultimately conquered by 652 AD. Heraclius himself would lose half of his Empire and a large portion of his army at Yarmouk before dying.
Ultimately, one may look at Nineveh as pointing out that it was kind of an inconsequential battle, after all, it couldn’t change the geopolitical situation of the world in any significant way that could have altered the Islamic conquests. However, I would say that this was the most consequential out of all the inconsequential battles. Ultimately, it was the last battle between the Iranians and the Romans, who had been rivals for centuries. But moreover, it was the last battle of Antiquity, a dirge for the era preceding it.