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The two great Arab military victories of 636, the Battle of Yarmouk against the Byzantines and the Battle of al-Qadisiyah against the Sasanian Persians, were decisive in both in purely military and in geopolitical terms. Taken individually, they changed the course of both of the defeated empires—the Byzantines would never again hold significant territory south of Anatolia, and the Sasanians would never again hold, well, pretty much anything. Taken together, they changed the course of world history, supplanting both empires—the regional superpowers of their day—with an upstart new empire based around an emerging religious movement known as "Islam."
A depiction of the Battle of al-Qadisiyah from an illustrated manuscript of the Shahnameh (Wikimedia Commons)
The Sasanians were already in bad shape before Qadisiyah, owing to their recent defeat in their 602-628 war against the Byzantines and to the political and social turmoil that followed. The war ended after the Sasanian ruler Khosrow II was overthrown by his nobles and replaced by his son, Kavadh II. An outbreak of plague further weakened the already battered empire, both due to the high loss of life and because one of the victims was Kavadh, who died only a few months into his reign. His death kicked off a civil war that finally ended when the 8 year old grandson of Khosrow II, Yazdegerd III (d. 651), was crowned in 632. The new emperor was obviously young to rule in his own right, and so the two nobles who saw to his enthronement, Rostam Farrokhzad and Piruz Khosrow, held the real power in the empire.
Khalid b. al-Walid's 633 campaign in Iraq (Mohammad Adil via Wikimedia Commons)
In contrast, the Rashidun Caliphate was doing very well for itself. The end of the Ridda Wars in 633 brought all of the Arabian peninsula under caliphal control, so in April of that year, with nowhere to go but north, then-Caliph Abu Bakr sent an army under his most capable general, Khalid b. al-Walid (d. 642), to conquer Iraq. Khalid had dramatic success, defeating the Persians in a series of battles that, by the end of the year, left him and his forces in control of virtually every part of Iraq outside of the Persian capital, Ctesiphon, which was located just southeast of where Baghdad is situated today. The Persians got a reprieve when, in December, Abu Bakr ordered Khalid to go to Syria to assume command of the much less successful Arab invasion of that Byzantine province. And as 634 wore on, the Persians began to take back some of their lost territory and defeated the Arabs in a handful of battles.
In 635, Yazdegerd and the Byzantine Emperor Heraclius hatched a scheme to put down this Arab uprising once and for all: a union of the two great empires sealed with a marriage between Yazdegerd and Heraclius's daughter (which would, conveniently for the Byzantines, leave Heraclius in the dominant position as Yazdegerd's father in-law). Both emperors planned to amass large armies to attack the Arabs simultaneously in both Syria and Iraq. The Arabs, who didn't have enough men to sustain two large-scale offensives and were moving troops between Iraq and Syria as threats dictated, surely wouldn't be able to handle major counter-offensives on both fronts at the same time, right?
Well, they didn't have to find out, because Heraclius launched his offensive in May 636, and Yazdegerd...wasn't ready yet. The caliph, Umar b. al-Khattab, delayed the Sasanian mobilization through the clever use of Muhammad's war-making precepts. He sent reinforcements to bolster his army in Syria while also sending a man named Saʿd ibn Abi Waqqas with a few thousand troops to Iraq, where in July he camped near the town of Qadisiyah, near Kufa, and...that's about it. Saʿd's job was to engage Yazdegerd in diplomacy, to offer the Persians the chance to avoid battle either by converting to the new (and still nebulous) Islamic faith, or agreeing to pay the poll tax required of non-Muslims. This had been standard procedure for Islamic armies since the time of Muhammad, who always offered his foes the chance to surrender before battle. The hope, of course, was that the Persians would surrender, but Umar presumably realized that was a long shot. But even if they didn't, the time spent talking meant that the Arabs, with their armies stretched thin between Syria and Iraq, could focus on Syria without having to worry about defending Iraq.
Yazdegerd prepared his army, under the command of Rostam Farrokhzad, and ordered it to march out to meet the Arabs, but, sure enough, he just left it sitting there for months, negotiating, until after the Arabs had won at Yarmouk and could turn their full attention to Iraq. Umar quickly ordered his commanders in Syria to dispatch men to join the army in Iraq, and they sent a force of 5000 now quite seasoned soldiers to the east. Even with those reinforcements, the Arabs were vastly outnumbered, with somewhere around 30,000-35,000 men against a Persian army that may have been as large as 100,000 (though it was probably somewhere between that figure and lower estimates of 50,000). Some sources put the Persian army as large as 200,000+, which seems excessive, but this was an army raised in self-defense by a very large empire, fighting on its home turf, so if any late antique army would have been that large, it would've been this one.
Arab expansion under Muhammad and the Rashidun caliphs (through 661); look closely at Iraq and you'll see Qadisiyah
Later Arab historians spend as much time recounting the talks between the Arabs and Persians as they do on the battle itself, but their accounts can't be considered accurate portrayals of real events. Many of them were written much later than the events themselves, and even the ones that were written within a relatively short time after those events are so stylized (including accounts of deliberations within the Persian court, featuring full dialogue) that they're obviously fictional. You read about ragged-looking Arab soldiers being brought to the immaculate, wealthy Persian court, where the Persians attempt to dazzle them with their vast riches to no effect, while the Persians are themselves dazzled by the fighting prowess and/or innate character of the Arab warriors. These are obviously stories rather than histories. You see some of the same tropes about the decadent Sasanians as you might find in an ancient Greek account of the decadent Achaemenids, or as you might later about the decadent Caliphal court in accounts of the Mongol conquest of Baghdad in 1258. They are valuable not as accounts of the actual negotiations between the Arabs and Persians, but as accounts of how the Arab conquerors and their descendants viewed the Persian Empire.
Finally, on November 16 (we think), the fighting started. As with the accounts of the Persian-Arab negotiations, we don't really know very much about the battle because most of what was written about it was written much later and in an episodic, kind of myth-building way (lots of champion-vs-champion fighting, for example). It would be like extrapolating details about the Trojan War, which probably was a real thing that really happened (maybe more than once), from reading the Iliad, which is a chronicle of heroic legends about mythic figures and was never intended to accurately describe real events. You can get a sense of the contours of the battle, but not the gory details.
The battle was a four-day affair (maybe; it's not really a lock that we're commemorating the right day here, just FYI), and a pretty brutal one, as neither side could really get an edge. The Arabs focused initially on countering the Persian war elephants, which they did unfortunately at great cost to the elephants (and to the men who got trampled as the hacked-up animals fled). On the second day, the reinforcements from Syria finally arrived, organized as a long stream of small units to fool the Persians into thinking there were more of them than there really were—7th century psychological warfare. At some point near the end of the battle, Rostam was apparently killed in an Arab strike on his command post. The decisive factor, again according to the sources, may have been the superiority of Arab archery and/or armor—their arrows could apparently penetrate the Persians' armor much more easily than vice versa. There may also have been a sandstorm blowing in the Persians' faces on the final day, but don't quote me on that.
Whatever finally caused the Persian army to break, it did break, and the way was open for the Arabs to take the last two prizes left in Persian Iraq. First was the Sawad, the rich alluvial plain between the Tigris and Euphrates Rivers in southern Iraq, which, along with the Nile Valley, became the breadbasket of the caliphate—at least for a couple of centuries until over-farming and over-irrigation began to ruin its soil. Second was Ctesiphon, which fell to the Arabs in March 637. The Persian Empire wasn't finished yet, but like Carthage after the Second Punic War, it was all over but the shouting. Umar decided to pause the conquests in order to consolidate control over the territories that the Arabs had just won, but when Yazdegerd attempted to reconquer Iraq in 642 (his army was quickly defeated at the Battle of Nahavand), that helped spur a new round of Arab expansion.
Yazdegerd spent the next several years basically running for his life throughout Iran, trying to raise some kind of army. But very few of Yazdegerd's governors in the east wanted to help him and risk getting on the wrong side of the oncoming Arab armies, and on the rare occasions when he could raise a force it would be quickly defeated. He wound up in the city of Merv, along the Central Asian Persian-Turkic frontier, in about 651. The governor (marzban) there, a guy named Mahuy or Mahuy Suri, wanted nothing to do with Yazdegerd, so he engaged a nearby Turkic tribe to kill him. Yazdegerd supposedly got wind of this plot and tried to flee, but was murdered by someone who may have been a simple thief (though it's also possible that Mahuy put the guy up to it) at an oasis outside the city. Yazdegerd's sons kept running east, eventually ending up in the Chinese court, still claiming to be the rightful rulers of the Persian Empire. But for all intents and purposes their empire died at Qadisiyah.
The fall of Constantinople was an astonishing event in the sense that there had been a "Roman Empire" of some kind or another in existence for nearly 1500 years, and nearly 2000 years if you include the Roman Republic. Likewise, if you go back to the Median Empire of the late 8th century BCE, there was a "Persian Empire" of some kind or another in existence for nearly 1400 years before the Arabs snuffed it out. The conquest of Persia expanded the caliphate and imperial Persia lent the new empire many of its high-court trappings. The Persian language, in a modified form, eventually became one of the great languages of Islamic civilization, and Persian culture came to define much of the Islamic world. Oh, and the conquest of Persia may have cost Umar his life. The caliph was assassinated in 644 by a former Persian soldier turned slave. The assassin’s motives are unclear, but revenge for the fall of his empire may have been among them.