Beginning in the 600s, the Middle East was a theater of war for three of history’s greatest empires. Two of them, the Byzantine and the Sassanid Empires, had battled for centuries and were descendants of empires that had stood even longer ago, Rome and Parthia, respectively. A new empire began to form, however, during the life of the Prophet Muhammad. As more and more converts joined his faith, the power of Islam grew out of the western part of Arabia and expanded quickly. When the Byzantines and Sassanids noticed this, they set aside their own differences and began an alliance for mutual protection.
The Sassanid Empire faced its own turmoil. Khosrau II was assassinated by his son Kavadh II in 629, who died in a matter of months, leading to a string of usurpations. Seven-year-old Ardashir III reigned before being killed by General Farrokhan, who died in battle and was succeeded by Purandokht, daughter of Khosrau II. She would rule for a short time, repairing much of the damage done by the past years’ intrigue before being replaced by her sister Azarmidokht, who would in turn be replaced by the nobleman Hormizd VI. Finally, Purandokht’s son and Khosrau’s grandson Yazdgerd III came of age and stabilized the Sassanid throne, supported by his general Rostam Farrokhzād.
Meanwhile, Muslim power continued to grow. Upon the death of the Prophet in 632, a council met and determined that Abu Bakr would be caliph. He set upon a series of wars uniting the Arabs of Arabia and then moving northward to add those in Syria and Palestine. With a new force the Middle East to counteract the tentative balance between Byzantium and Persia, wars quickly began with the caliphate invasion of Iraq, and the Muslim power was affirmed with the defeat of an army Sassanids, Byzantines, and Christian Arabs in 633. Sassanids finally stopped the Muslim advance in 634, and Sassanids and Byzantines made a formal alliance in 635. Knowing of the alliance, the Muslim forces decided to deal with their enemies one at a time, wiping out the Byzantine army at Yarmouk near the Sea of Galilee in August of 636 at the cost of abandoning Iraq to a massive Sassanid force of some 200,000 in camp. The Muslims camped at Qādisiyyah with 30,000 and waited as peace talks dragged on.
That November, the talks gave way to actual battle. The two sides had attempted to bend the other’s will with the Muslims sending an emissary to convert Yazdgerd III while the Persians sent a Muslim ambassador home as a servant carrying a basket of dirt on his head (though the Muslim response was, “Congratulations! The enemy has voluntarily surrendered its territory to us”). Caliph Umar ordered the talks to end, which caused Sassanid General Rostam to prepare for battle despite his reservations. Although the Sassanid army was much larger, the vast majority of the force was conscripted spear infantry that Procopius of Caesarea described as "a crowd of pitiable peasants who come into battle for no other purpose than to dig through walls and to despoil the slain and in general to serve the [real] soldiers.”
In the night, Rostam decided to use the infantry, what might have been his weakness, as a diversion. He dammed up the canal and moved over his entire army to face the Muslim force the next morning. Following secret orders, the infantry led the attack as a whole after the opening onslaught of the arrows, but were swiftly beaten back by the better trained Muslims. They feigned retreat, and the Muslims pursued. When they reached the canal, however, the infantry turned about and were ordered to hold position while the archers pounded the Muslims, who then began their own retreat. In the chaos (the battle would be known to Islam as Yaum-ul-Armah, "The Day of Disorder"), Rostam released his war elephants, backed by his heavy cavalry, which swept the Muslim cavalry from the field. The organized retreat turned to a rout with the unnerving elephants stomping, and the Muslim army was destroyed.
Yazdgerd III would manage to seal the victory with a treaty that would end his alliance with the now extremely weakened Byzantine Empire. The Zagros Mountains were strongly defended against further Muslim invasion, though the rich lands of Mesopotamia would routinely change hands between them, like Anatolia, which would be stripped from the Byzantines, who became a relic city-state with a naval empire. The Muslim caliphates, meanwhile, would turn westward, conquering across North Africa and into Europe, where Christians would begin counterattacks such as the Crusades with Persian allies.
While the political boundaries settled for the time, the wildly different religions of the Arab and Persian peoples would keep up a constant sense of distrust. Although conquered by the Mongols and later European colonialists, Persia would remain the center of Zoroastrianism, one of the world’s largest religions.
In reality, the Battle of al-Qādisiyyah lasted four days. Fighting was bloody but inconclusive until the fourth day, when Rostam’s war elephants were driven from the battle for good. Rostam was killed in the battle, and his army disintegrated without a leader, giving a great victory to the Muslims, who would soon conquer Persia and add it to the Rashidun Caliphate. After centuries of religious persecution, only a handful of Zoroastrians remain, and Iran is the seat of Shia Islam.