Sasanian Empire: The Rise and Fall of An Empire by Touraj Daryaee

 Rise of the Sasanian Empire:

The Sasanian Empire, officially known as the Empire of Iranians Eranshahr and called the Neo-Persian Empire by historians, was the last Persian imperial dynasty before Islam arrived in the mid-seventh century AD. Named after the House of Sasan, it endured for over four centuries, from 224 to 651 AD, making it the longest-lived Persian dynasty. The Sasanian Empire succeeded the Parthian Empire and reestablished the Iranians as a superpower in late antiquity, alongside its neighboring archrival, the Roman-Byzantine Empire.

Ardashir I founded the Sasanian Empire, a local Iranian ruler who rose to power as Parthia weakened from internal strife and wars with Rome. After defeating the last Parthian shahanshah, Artabanus IV, in the battle of Hormozdgan in 224, he established the Sasanian dynasty and set out to restore the legacy of the Achaemenid Empire by expanding Iran's dominions. At its greatest extent, the Sasanian Empire encompassed all of present-day Iran and Iraq. It stretched from the eastern Mediterranean (including Anatolia and Egypt) to Pakistan and from southern Arabia to the Caucasus and Central Asia. According to legend, the vexilloid of the Empire was the Derafsh Kaviani.

The period of Sasanian rule is considered a high point in Iranian history. In many ways, it was the peak of ancient Iranian culture before the Muslim conquest and subsequent Islamisation. The Sasanians tolerated their subjects' varied faiths and cultures; developed a complex, centralized government bureaucracy; revitalized Zoroastrianism as a legitimizing and unifying force of their rule; built grand monuments public works; and patronized cultural and educational institutions. The Empire's cultural influence extended far beyond its territorial borders—including Western Europe, Africa, China, and India—and helped shape European and Asian medieval art. Persian culture became the basis for much of Islamic culture, influencing art, architecture, music, literature, and philosophy throughout the Muslim world.

The Perpetual Peace, signed in 532 between the East Roman (Byzantine) Empire and Sassanid Persia, was a peace treaty of indefinite duration, which concluded the Iberian War (527–531) between the two powers. It heralded a period of relatively cordial relations but lasted only until 540 when hostilities resumed over the control of Lazica.

When Persian King Kavadh I passed away (r. 488–531) in late 531, his third son Khosrau I (r. 531–579) ascended to the throne. The dynamic of the situation changed: Khosrau began to face domestic insecurity. The Byzantine Ruler, Emperor Justinian I (r. 527–565), was contemplating recovering the lost western half of the Roman Empire than pursuing a war against Persia. The Byzantine envoys Rufinus found Khosrau more conciliatory than his father. The Byzantine Ruler Justinian and an agreed to pay 11,000 pounds or 5000 Kilograms of gold, ostensibly contributing to the Caucasus's defense, passes against the barbarians living beyond. The base of the Mesopotamia would be withdrawn from the fortress of Dara to the city of Constantina. The two rulers would recognize once again each other as equal and pledged mutual assistance. Khosrau initially refused to hand back the two Lazic forts while demanding the return of the two other forts the Byzantines had captured in Persian Armenia. Justinian at first agreed but soon changed his mind, causing the agreement to be broken off. In summer 532, however, Hermogenes and Rufinus's new embassy managed to persuade Khosrau for a full exchange of the occupied forts and allow the exiled Iberian rebels to either remain in the Byzantine Empire or return unmolested to their homes.

Fall of the Sasanian Empire:

Khosrau, in coordination with Avar and Slavic forces, launched a siege on the Byzantine capital of Constantinople in 626. The Sassanids, led by Shahrbaraz, attacked the city on the Bosphorus's eastern side, while his Avar and Slavic allies invaded from the western side. The Byzantine fleet blocked attempts to ferry the Persian forces across the Bosphorus to aid their allies, the Slavic troops being by far the most capable in siege warfare, the siege failed. In 627–628, Heraclius mounted a winter invasion of Mesopotamia, and, despite the departure of his Khazar allies, defeated a Persian army commanded by Rhahzadh in the Battle of Nineveh. He then marched down the Tigris, devastating the country and sacking Khosrau's palace at Dastagerd. He prevented attacking Ctesiphon by destroying the Nahrawan Canal bridges and conducted further raids before withdrawing up the Diyala into north-western Iran.

Queen Boran, daughter of Khosrau II, the first woman and one of the last rulers on the throne of the Sasanian Empire, reigned from 17 June 629 to 16 June 630. 

The impact of Heraclius's victories, the devastation of the richest territories of the Sassanid Empire, and the humiliating destruction of high-profile targets such as Ganzak and Dastagerd fatally undermined Khosrau's prestige and his support among the Persian aristocracy. In early 628, he was overthrown and murdered by his son Kavadh II (628), who immediately brought an end to the war, agreeing to withdraw from all occupied territories.

 In 629, Heraclius restored the True Cross to Jerusalem in a majestic ceremony. Kavadh died within months, and chaos and civil war followed. Over four years and five successive kings, the Sassanid Empire weakened considerably. The power of the central authority passed into the hands of the generals. It would take several years for a strong king to emerge from a series of coups, and the Sassanids never had time to recover fully.

In early 632, a grandson of Khosrau I, who had lived in hiding in Estakhr, Yazdegerd III, acceded to the throne. The same year, the first raiders from the Arab tribes, newly united by Islam, arrived in Persian territory. Years of warfare had exhausted both the Byzantines and the Persians. The Sassanids were further weakened by economic decline, heavy taxation, religious unrest, rigid social stratification, the increasing power of the provincial landholders, and a rapid turnover of rulers, facilitating Persia's Islamic conquest.