Praise be to Allah.
Before the Islamic conquest, Iran was under the rule of the Sasanids, and suffered from a great deal of injustice, corruption and oppression. Zoroastrianism was the official religion of Iran until the Islamic conquest.
The Islamic conquest of Iran began during the caliphate of Abu Bakr as-Siddeeq (may Allah be pleased with him), when al-Muthanna ibn Haarithah conquered some of the neighbouring lands in the region of Mesopotamia in 13 AH.
During the caliphate of ‘Umar (may Allah be pleased with him), Abu ‘Ubayd ath-Thaqafi (may Allah be pleased with him) tried to invade Iran from the south-west, and faced the Persians in the Battle of the Bridge in 13 AH. He met Jaabaan in battle, and Allah defeated the Magians and enabled the Muslims to capture Jaabaan. Then Abu ‘Ubayd marched to Kaskar when he and Nursi met in battle and he defeated him. Then he met Jalinus in battle and defeated him. Then the tide of fortune turned against the Muslims, and many of them were killed and drowned, and Abu ‘Ubayd (may Allah be pleased with him) was killed.
Siyar A‘laam an-Nubala’ (2/421)
Then the Muslims completed the conquest of Iran under the leadership of Sa‘d ibn Abi Waqqaas in 14 AH, when the Muslim army in Persia inflicted a mighty defeat (on the Persians) in the Battle of al-Qaadisiyyah, which was one of the major battles of the Muslims.
The historians stated that the Muslims were between seven and eight thousand strong, and that Rostom, the Persian commander, had sixty thousand troops.
On a Monday in Muharram 14 AH, a strong wind blew and lifted the tents of the Persians from their places, and the couch of Rostom, that had been set up for him, was overturned. He hastened to ride his mule and flee, but the Muslims overtook him and killed him. They also killed al-Jalinus, the commander of the Persian vanguard, and the Persians were defeated – praise be to Allah – to the last man and the Muslims pursued them. On that day those who were chained were killed in their entirety, of whom there were thirty thousand. Ten thousand were killed in the battle, and before that a similar number had also been killed. On that day and on the previous days, two thousand five hundred Muslims were killed, may Allah have mercy on them.
The Muslims pursued those who fled until they entered the capital city behind them, namely al-Madaa’in (Ctesiphon), which was the seat of the court of Chosroes.
From the battle of al-Qaadisiyyah the Muslims seized an indescribably huge amount of wealth and weapons. The booty was gathered after giving the troops their shares, and the khums and glad tidings of the victory were sent to Ameer al-Mu’mineen ‘Umar ibn al-Khattaab (may Allah be pleased with him).
Al-Bidaayah wa’n-Nihaayah (9/630)
Then the Muslims continued their advance into Persian territory, and were enabled to conquer the southern part of Iran, whilst the ruler Yazdegerd III retreated to the region of Isfahan in central Iran, where he began to assemble troops in an attempt to retake what he had lost. The Muslims and the troops of Yazdegerd met in the Battle of Jalula in 18 AH, which ended with the defeat of Yazdegerd and his army, and his retreat towards Isfahan.
At-Tabari (may Allah have mercy on him) said:
On that day Allah killed one hundred thousand of them, and the slain covered [jallalat] the ground in all directions. It was called Jalula because it was covered with the bodies of their slain.
Tareekh at-Tabari (4/26)
Then Yazdegerd met the Muslims in the decisive Battle of Nahawand in 21 AH, where the Muslims won a clear victory, after which the Sasanids no longer had any power. The Muslims seized a great deal of booty, which led to them dubbing this battle “the conquest of conquests”.
Ibn Katheer (may Allah have mercy on him) said: It was a mighty battle of great significance and wondrous events, and the Muslims used to call it the conquest of conquests. End quote.
Al-Bidaayah wa’n-Bihaayah (10/111)
It took the Muslims ten years to gain control over all the Iranian lands, because of their huge size and difficult terrain.
One of the things that played a role in the spread of Islam in Iran was the migration of many Arab tribes to Iranians land in which they settled and mixed with the Iranians.
For nearly nine centuries, Iran followed the madhhab of Ahl as-Sunnah wa’l-Jamaa‘ah, until the Shi‘i Safavid state seized control of it in 906 AH.
Tareekh at-Tabari by Ibn Jareer at-Tabari
Siyar A‘laam an-Nubala’ by al-Haafiz Shams ad-Deen adh-Dhahabi
Al-Bidaayah wa’n-Nihaayah by al-Haafiz Ibn Katheer ad-Dimashqi
Al-Mufassal fi Tareekh al-‘Arab qabl al-Islam by Dr Jawaad ‘Ali
Iran fi Zill al-Islam fi’l-‘Usooi as-Sunniyyah wa’sh-Shi‘iyyah by Dr ‘Abd al-Mun‘im Hasanayn
Qaadat Fath Bilaad Faaris – Mahmoud Sheet Khattaab
Iran wa Afghanistan – Mahmoud Shaakir
Iran mundhu Fajr at-Tareekh hatta al-Fath al-Islami by Dr Muhammad ‘Abd al-Qaadir Muhammad
And Allah knows best.