As part of the early Muslim conquests, which had begun under Muhammad in 622, it led to the fall of the Sasanian Empire and the eventual decline of Zoroastrianism, which had been predominant throughout Persia as the nation's official religion.

While Arabia was experiencing the rise of Islam in the 7th century, Persia was struggling with unprecedented levels of political, social, economic, and military weakness; the Sasanian army had greatly exhausted itself in the Byzantine–Sasanian War of 602–628.

The second Rashidun invasion began in 636, under Sa'd ibn Abi Waqqas, when a key victory at the Battle of al-Qadisiyyah permanently ended all Sasanian control to the west of modern-day Iran.

Directing the war from the city of Medina in Arabia, Umar's quick conquest of Persia in a series of coordinated and multi-pronged attacks became his greatest triumph, contributing to his reputation as a great military and political strategist.

Many localities fought against the invaders; although the Rashidun army had established hegemony over most of the country, many cities rose in rebellion by killing their Arab governors or attacking their garrisons.

[10] Despite their recent victories over the Byzantine Empire, the Parthians unexpectedly withdrew from the confederation, and the Sassanians were thus ill-prepared and ill-equipped to mount an effective and cohesive defense against the Muslim armies.

Pourshariati argues that the Arab conquest of Mesopotamia "took place, not, as has been conventionally believed, in the years 632–634, after the accession of the last Sasanian king Yazdgerd III (632–651) to power, but in the period from 628 to 632.

Over a period of fourteen years and twelve successive kings, the Sassanid Empire weakened considerably, and the power of the central authority passed into the hands of its generals.

From 612 to 622, he extended the Persian borders almost to the same extent that they were under the Achaemenid dynasty (550–330 BC), capturing Western states as far as Egypt, Palestine (the conquest of the latter being assisted by a Jewish army), and more.

[19] With regards to Persia, Muslim histories further recount that at the beginning of the seventh year of migration, Muhammad appointed one of his officers, Abdullah Huzafah Sahmi Qarashi, to carry his letter to Khosrau II inviting him to convert: In the name of God, the Beneficent, the Merciful.

[10] After the last Sasanian-Byzantine war, the Parthians wanted to withdraw from the confederation, and the Sasanians were thus ill-prepared and ill-equipped to mount an effective and cohesive defense against the Muslim armies.

Pourshariati argues that the Arab conquest of Mesopotamia "took place, not, as has been conventionally believed, in the years 632–634, after the accession of the last Sasanian king Yazdgerd III (632–651) to power, but in the period from 628 to 632.

The Persians, who had only one generation before conquered Egypt and Asia Minor, lost decisive battles when nimble, lightly armed Arabs accustomed to skirmishes and desert warfare attacked them.

After the Ridda wars, a tribal chief of northeastern Arabia, Al-Muthanna ibn Haritha, raided the Sasanian towns in Mesopotamia, actions that generated a considerable amount of booty was collected.

Khalid received a call for aid from northern Arabia at Dawmat al-Jandal, where another Muslim Arab general, Iyad ibn Ghanm, was trapped among the rebel tribes.

While Khalid was on his way to attack Qadissiyah (a key fort en route to Ctesiphon), Abu Bakr ordered him to the Roman front in Syria to assume command there.

During Abu Bakr's era, Khalid ibn al-Walid had left Mesopotamia with half his army of 9000 soldiers to assume command in Syria, whereupon the Persians decided to take back their lost territory.

In 635 Yazdgerd III sought an alliance with Emperor Heraclius of the Eastern Roman Empire, marrying the latter's daughter (or, by some traditions, his granddaughter) in order to seal the arrangement.

While Heraclius prepared for a major offence in the Levant, Yazdegerd ordered the concentration of massive armies to push the Muslims out of Mesopotamia for good through a series of well-coordinated attacks on two fronts.

Umar, allegedly aware of this alliance and not wanting to risk a battle with two great powers simultaneously, quickly reinforced the Muslim army at Yarmouk to engage and defeat the Byzantines.

[36] After the defeat of the Persian forces at the Battle of Jalula in 637, Yazdgerd III went to Rey and from there moved to Merv, where he set up his capital and directed his chiefs to conduct continuous raids in Mesopotamia.

Accordingly, he recruited 100,000 hardened veterans and young volunteers from all parts of Persia, under the command of Mardan Shah, which marched to Nahavand for the last titanic struggle with the Caliphate.

After the devastating defeat at Nahavand, the last Sassanid emperor, Yazdegerd III, fled to different parts of Persia to raise a new army, with limited success, while Umar attempted to capture him.

Umar chose Isfahan, as it was the heart of the Persian Empire and a conduit for supply and communications among the Sasanian garrisons, and its capture would isolate Fars and Azerbaijan from Khorasan, Yazdegerd's stronghold.

All the commanders, before starting their assigned campaigns, were instructed to send a detailed report of the geography and terrain of the region and the positions of the Persian garrisons, forts, cities and troops.

In 651, Nu'aym ibn Muqaarin, Nu'man's brother, marched northeast to Rey, Iran, about 320 kilometres (200 mi) from Hamadan, and laid siege to the city, which surrendered after fierce resistance.

The Muslim invasion of Fars began in 638/9, when the Rashidun governor of Bahrain, al-'Ala' ibn al-Hadrami, having defeated some rebellious Arab tribes, seized an island in the Persian Gulf.

This horrified Aparviz, who, in order to spare the inhabitants of Sakastan from the Arabs, made peace with them in return for a heavy tribute of 1 million dirhams, including 1,000 slave boys (or girls) bearing 1,000 golden vessels.

[54] Acting upon a claim that they had been seen conspiring with Abu Lu'lu'a while he was holding the murder weapon, Ubayd Allah killed Hurmuzān, an ex-Sasanian military officer who had been working for Umar as an adviser after his capture by the Muslims, and Jufayna, a Christian man from the city of Al-Hira in Iraq who had been taken to Medina to serve as a tutor.

[57] Finally, in 651, Yazdegerd III, the last Sassanid emperor, was killed near Merv by a local miller for his purse,[58] thus putting an end to both his dynasty and to organized Persian resistance.