The Arab conquest of Sassanian Iran took place in the middle of the 7th century and put an end to the existence of the Sassanid state in 644 and also led to the decline of the religion of Zoroastrianism in Iran, although the Sassanid dynasty finally fell in 651 when the last heir to the throne was killed. This conquest also led to a significant decrease in the influence of the Zoroastrian religion on the territory of the so-called Greater Iran and its almost complete disappearance later.
There is a true story that in 628 the Prophet Mohammed sent the Byzantine Emperor Heraclius I, the Iranian Shah Khosrow II Parviz and other rulers of the neighboring states letters asking to convert to Islam. The reaction of the Byzantine emperor is unknown, Khosrow was enraged by the inappropriate treatment of him (“In the name of Allah, Most Gracious, Most Merciful…) and tore up the letter. When the prophet found out about this, he said that Allah would tear apart the kingdom of Khosrow just as Khosrow had torn this letter.
Muslims first appeared in the Sassanian territories in 633, when, under the leadership of the great commander Khalid ibn Walid, they invaded Mesopotamia, which together with Babylonia was the “apple of discord” between the Roman Empire and Sassanian Iran in the era of the Persian-Roman wars.
The first blow of the Muslims in 633, the Persians could not repel and suffered a series of defeats partly because they were weakened by the wars with Byzantium. Nevertheless, the Sassanid Empire still remained a great power, owning lands from Asia Minor to India.
The second invasion began in 636 under the leadership of Saad ibn Abu Wakkas, whose troops defeated the Persians in the “key” campaign of the Battle of al-Qadisiyyah, which became one of the most significant events in the history of the East and was actually the beginning of the transition from Antiquity to the Early Middle Ages.
The victory of the Muslims in the battle of al-Qadisiyyah led to the creation of a new Arab-Persian border – in the region of the Zagros ridge.
The Persians did not reconcile with the new status quo in the region and several times tried to return the Mesopotamia conquered by Muslims; that is why caliph Umar launched a large-scale offensive in 642 with the goal of completely conquering Sassanian Iran. In 642, the decisive battle of Nahavand, in which the Persian army was defeated, took place.
The conquest of Sassanid Iran was actually completed in 652 with the death of the last Shah Yezdigerd III. Such a rapid conquest of Persia was the result of the defeat in the Iranian-Byzantine war in 628, which some historians call the “world war of the 7th century,” so much financially and economically affected almost all the states and territories of the Mediterranean and the Middle East.
Nevertheless, some Iranian historians, referring to Arab sources, paint a different picture of the conquest of Persia – with fierce resistance of the local population to the Arabs. Despite the fact that by the second half of VII century A.D. Islam became the dominant religion in Iran (although Zoroastrianism was never completely destroyed), the majority of the region’s population continued to be constituted by indigenous peoples, who were despicable of Arab culture.
The consequence of the Arab conquest of Persia is the further spread of Islam to the East: it was from there that it spread to the territory of the Volga Bulgaria, the Uyghurs, and then some countries of Southeast Asia.
Some southern regions of the former Sassanian state offered the most stubborn resistance to the Arab conquerors and were actually independent of the central government for a long time after 644.