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Decline and fall (622–651)

While successful at its first stage (from 602 to 622), the campaign of Khosrau II had actually exhausted the Persian army and treasuries. In an effort to rebuild the national treasuries, Khosrau overtaxed the population. Thus, while his empire was on the verge of total defeat, Heraclius (610–641) drew on all his diminished and devastated empire's remaining resources, reorganised his armies, and mounted a remarkable, risky 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 Shahrbaraz, Shahin, and Shahraplakan (whose competition to claim the glory of personally defeating the Byzantine emperor contributed to their failure), sacking the great Zoroastrian temple at Ganzak, and securing assistance from the Khazars and Western Turkic Khaganate.

In response, 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 eastern side of the Bosphorus, while his Avar and Slavic allies invaded from the western side. Attempts to ferry the Persian forces across the Bosphorus to aid their allies (the Slavic forces being by far the most capable in siege warfare) were blocked by the Byzantine fleet, and the siege ended in failure. 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 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. 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 a period of 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. According to Howard-Johnston, 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 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, were no longer a threat. Caliph Abu Bakr 's commander Khalid ibn Walid, once one of Muhammad 's chosen companions-in-arms and leader of the Arab army, moved to capture Iraq in a series of lightning battles. Redeployed to the Syrian front against the Byzantines in June 634, Khalid's successor in Iraq failed him, and the Muslims were defeated in the Battle of the Bridge in 634. However, the Arab threat did not stop there and reemerged shortly via the disciplined armies of Khalid ibn Walid.

In 637, a Muslim army under the Caliph Umar ibn al-Khattāb defeated a larger Persian force led by General Rostam Farrokhzad at the plains of al-Qādisiyyah, and then advanced on Ctesiphon, which fell after a prolonged siege. Yazdegerd fled eastward from Ctesiphon, leaving behind him most of the empire's vast treasury. The Arabs captured Ctesiphon shortly afterward. Thus the Muslims were able to seize a powerful financial resource, leaving the Sassanid government strapped for funds. 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 Nihawānd. 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 Arab invaders.

Upon hearing of the defeat in Nihawānd, Yazdegerd along with Farrukhzad and some of the Persian nobles fled further inland to the eastern province of Khorasan. Yazdegerd was assassinated by a miller in Merv in late 651. His sons, Peroz and Bahram, fled to Tang China. Some of the nobles settled in Central Asia, where they contributed greatly to spreading the Persian culture and language in those regions and to the establishment of the first native Iranian Islamic dynasty, the Samanid dynasty, which sought to revive Sassanid traditions.

The abrupt fall of the Sassanid Empire was completed in a period of just 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. Islamic caliphates repeatedly suppressed revolts in cities such as Rey, Isfahan, and Hamadan. The local population was initially under little pressure to convert to Islam, remaining as dhimmi subjects of the Muslim state and paying a jizya. In addition, the old Sassanid "land tax" (known in Arabic as Kharaj) was also adopted. Caliph Umar is said to have occasionally set up a commission to survey the taxes, to judge if they were more than the land could bear.