The Sassanid Empire was one of the strongest empires in the world in Late Antiquity and was able to hold its own with the Romans and later the Byzantine Empire. However, this mighty Zoroastrian empire fell quickly to the Muslims by 654 AD. Why did it fall so easily when it had resisted Roman expansion in spite of its capital being captured and burned by the Romans multiple times? The most important reason is that the empire was wrecked by defeat at the hands of Heraclius and Civil War.
The Sassanids and the victory of Heraclius.
The Byzantine Emperor Hercalius, one of the greatest Roman Emperors in history, managed to claw victory out of the jaws of defeat. The Sassanids, who had once come close to reuniting the classic empire of Xerxes and Darius had beaten at the Battle of Nineveh in 622 AD.
Khosrow II, the greatest of the Iranian conquerors and Shahs was overthrown and killed by his son, Kavad II. What occurred was an empire that was thrown into chaos and became incredibly weakened by the time of the Rashidun Caliphate’s conquests in the 630s AD.
When the Caliphate began attacking the Sassanids, the regime had already gone through many shahs, as many as ten men claiming the title of Shahanshah before Yazdegerd III would attempt to restore order. He took power just as the Muslims began expanding their empire.
The Sassanids had formidable military with their horse archers and heavy cavalry being powerful on the battlefield against the Romans. However, the Sassanid Empire had been suffering huge amounts of chaos. The nobles who had helped Kavad II rise to power began forming factions which warred against each other in the Sassanid realm.
While there had been civil wars and succession wars before, this one came at a time when the Empire was morally very weak after defeat by the Christian Byzantine Empire.
The Muslims, who had recently been united under Mohammed, were motivated and brought their own brand of military tactics as well as diplomatic zeal and cunning which allowed them to sweep over the Sassanid Empire within less than a generation.
Sassanid Iran fell for very much the same reasons the Byzantine Empire fell in the 15th century; competition between elites became too overbearing on the empire. These civil wars and machinations left Sassanid Iran vulnerable to the conquests of the Rashidun Caliphate.
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