The Arab conquest of Iran made way to end of the Sasanian Empire and the final deterioration of the Zoroastrianism in Iran. The growth of Muslims overlapped with an important political, societal, economic and martial weakness in Persia.
Once a major power, the Sasanian Empire lost its human and material resources after years of rivalry against the Byzantine Empire. The internal political situation immediately declined after the conviction of King Khosrau II. Consequently, ten new people were crowned within the next four years, highlighting the political instability of the Sassanians prior to the Muslim attack.
Arab Muslims first attacked the Sassanid territory in 633, when General Khalid Ibn Walid conquered Mesopotamia, which was the political and economic center of the Sassanid state. After the transfer of Khalid to the Byzantine front in the Levant, the Muslims subsequently lost their assets and belongings to Sassanian counterattacks.
The second attack was done in 636 under Saad ibn Abi Waqqas, wherein a significant triumph at the Battle of Qadisiyyah led to the permanent decline of Sasanian control west of Iran. The Zagros Mountains turned out to be a natural wall and border between the Rashidun Caliphate and the Sassanid Empire. As a consequence of regular raids by Persians into the area, Caliph Umar ordered a full attack on the Sasanian empire in 642, which led to the complete conquest of the Sasanians around 651 Directing from Medina, a few thousand kilometers from the battlefields of Iran, Caliph Umar’s quick conquest of Iran in a series of well-coordinated, multi-pronged attacks became his greatest win, enhancing his standing as a great military and political strategist
Iranian historians have protected their descents vis a vis Arab sources to demonstrate that “contrary to the claims of some historians, Iranians, in fact, fought long and hard against the conquering Arabs. By 651, most of the urban areas in Iranian lands, with the noteworthy exemption of the Caspian provinces (Tabaristan) and Transoxiana, had come under the power and control of the Arab armies. Many localities fought against the attackers; ultimately, none were successful.
In fact, although Arabs had established control over most of the country, many cities got up in revolt by killing the Arab governor and attacking their forts. Finally, military support suppressed the revolution and levied Islamic control. The violent suppression of Bukhara is a case in point: Conversion to Islam was gradual, partially as the result of this violent resistance; however, Zoroastrian scriptures were burnt and many priests were executed.
However, the Persians began to reassert themselves by maintaining the Persian language and culture. Regardless, Islam was adopted by many for a number of causes including by threat and extortion, for political and economic reasons, and/or by persuasion. Islam became the leading religion late in the medieval ages.
Strategic Planning for Conquest of Central Persia
The strategic planning of the takeover of the Persian Empire was completed by early 642. Umar appointed Abdullah ibn Uthman, commander of the Muslim forces, to attack Isfahan. Once Hamadan was captured, Nu’man marched 350+ kilometers southeast against the city Isfahan and defeated a Sasanian army under the command of Shahrvaraz Jadhuyih and other prominent Sasanian generals. Shahrvaraz Jadhuyih, along with another Sasanian general was killed during the attack.
After his success at Isfahan, he laid siege to the city; there the Muslim army was strengthened by fresh troops from Busra and Kufa under the command of Abu Musa Ashaari and Ahnaf ibn Qais. The restriction continued for a few months and finally, the city surrendered.
In 651, Nu’aym marched northeast to Rey, Iran, about 320 kilometres from Hamadan, and laid siege to the city, which was abandoned after fierce resistance. Nu’aym then marched 240 kilometres northeast towards Qom, which was captured without much effort. This was the outer border of the Isfahan region.
Meanwhile, Hamadan and Rey had protested. Umar sent Nu’aym ibn Muqaarin, brother of late Nu’man ibn Muqaarin, who was the Muslim commander at Nihawand, to crush the rebellion and to clear the westernmost boundaries of Isfahan. Nu’aymm marched towards Hamadan from Isfahan. Battle was fought and Hamadan was again captured by the Muslims. Nu’aym next moved to Rey.
There too the Persians resisted and were defeated outside the fort, and the city was recaptured by the Muslims. The Persian citizens sought for peace and agreed to pay the Jizya., Nu’aym moved north towards Tabaristan after Rey, which lay south of the Caspian Sea. The ruler of Tabaristan then signed a peace treaty with the Caliphate.