The replacement of the Sassanid regime by the Arab Caliphate armies.
Read and weep for the dead, for our situation is not so different from theirs.
Although hugely successful at first glance, Khosrau II’s campaign had in fact overextended the Persian army and overtaxed the people. The Byzantine emperor Heraclius (610E41) drew on all his diminished and devastated empire’s remaining resources, reorganised his armies and mounted a remarkable counter-offensive. Between 622 and 627 he campaigned against the Persians in Anatolia and the Caucasus, winning a string of victories against Persian forces under Khosrau, Shahrbaraz, Shahin and Shahraplakan, sacking the great Zoroastrian temple at Ganzak and securing assistance from the Khazars and Western Turkic Khaganate. In 626 Constantinople was besieged by Slavic and Avar forces which were supported by a Persian army under Shahrbaraz on the far side of the Bosphorus, but attempts to ferry the Persians across were blocked by the Byzantine fleet and the siege ended in failure. In 627-8 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 of Dastagerd. He was prevented from attacking Ctesiphon by the destruction of the bridges on the Nahrawan Canal and conducted further raids before withdrawing up the Diyala into north-western Iran.
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, and early in 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 AD Heraclius restored the True Cross to Jerusalem in a majestic ceremony. Kavadh died within months and chaos and civil war followed. Over a period of four years and five successive kings, including two daughters of Khosrau II and spahbod Shahrbaraz, 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 the spring of 632, a grandson of Khosrau I who had lived in hiding, Yazdegerd III, ascended 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. These factors facilitated the Islamic conquest of Persia.
The Sassanids never mounted a truly effective resistance to the pressure applied by the initial Arab armies. Yazdegerd was a boy at the mercy of his advisers and incapable of uniting a vast country crumbling into small feudal kingdoms, despite the fact that the Byzantines, under similar pressure from the newly expansive Arabs, no longer threatened. The first encounter between Sassanids and Muslim Arabs was in the Battle of the Bridge in 634 which resulted in a Sassanid victory, however the Arab threat did not stop there and reappeared shortly from the disciplined armies of Khalid ibn Walid, once one of Muhammad’s chosen companions-in-arms and leader of the Arab army. Under the Caliph `Umar ibn al-Khattab, a Muslim army defeated a larger Persian force lead by general Rostam Farrokhzad at the plains of al-Qadisiyyah in 637 and besieged Ctesiphon. Ctesiphon fell after a prolonged siege. Yazdgerd fled eastward from Ctesiphon, leaving behind him most of the Empire’s vast treasury. The Arabs captured Ctesiphon shortly afterward, leaving the Sassanid government strapped for funds and acquiring a powerful financial resource for their own use. A number of Sassanid governors attempted to combine their forces to throw back the invaders, but the effort was crippled by the lack of a strong central authority, and the governors were defeated at the Battle of Nihawand; the empire, with its military command structure non-existent, its non-noble troop levies decimated, its financial resources effectively destroyed, and the Asawaran (Azatan) knightly caste destroyed piecemeal, was now utterly helpless in the face of the invaders.
Upon hearing the defeat in Nihawand, Yazdgerd along with most of Persian nobilities fled further inland to the eastern province of Khorasan. He was assassinated by a miller in Merv in late 651 while the rest of the nobles settled in central Asia where they contributed greatly in spreading Persian culture and language in those regions and the establishment of the first native Iranian Islamic dynasty, the Samanid dynasty, which sought to revive and resuscitate Sassanid traditions and culture after the invasion of Islam.
The abrupt fall of Sassanid Empire was completed in a period of five years, and most of its territory was absorbed into the Islamic caliphate; however, many Iranian cities resisted and fought against the invaders several times. Cities such as Rayy, Isfahan and Hamadan were exterminated thrice by Islamic caliphates in order to suppress revolts. The local population either willingly accepted Islam, stayed as dhimmi subjects of the Muslim state and paid a poll tax ( jizya), or were forced to convert by the invading armies. The latter measure is usually disputed in its use though as most conversion took place primarily in the Abbasids caliphate. Invaders destroyed the Academy of Gundishapur and its library, burning piles of books. Most Sassanid records and literary works were destroyed. A few that escaped this fate were later translated into Arabic and later to Modern Persian. During the Islamic invasion many Iranian cities were destroyed or deserted, palaces and bridges were ruined and many magnificent imperial Persian gardens were burned to the ground. Persian poets such as Ferdowsi lamented the downfall of the Sassanids in their work:
Ibn Walid also took part in the conquering of Egypt, including Alexandria. The fact that there are different dates for when the Library of Alexandria burned, and one of them is during Walid’s march through Egypt, I would say the Arabs burned the Library of Alexandria given their historical behavior in the rest of their military conquests. Coincidentally, the Persian libraries were lost too. Perhaps too many coincidences happening at once.