The Arab conquest of Persia leads to the end of the Sassanid Empire in 644, the fall of Sassanid dynasty in 651 and the eventual decline of the Zoroastrian religion in Iran.

Arabs first enter Sassanid territory in 633, when general Khalid ibn Walid invades what is now Iraq.

Following the transfer of Khalid to the Roman front in the Levant, the Muslims eventually lose their holdings to Iranian counterattacks.

The second invasion begins in 636 under Saad ibn Abi Waqqas, when a key victory at the Battle of Qadisiyyah leads to the permanent end of Sassanid control west of Iran.

The Zagros mountains then become a natural barrier and border between the Rashidun Caliphate and the Sassanid Empire.

Owing to continuous raids by Persians into the area, Caliph Umar orders a full invasion of the Sassanid Iranian empire in 642, which is completed with the complete conquest of the Sassanids by mid 644.

The quick conquest of Iran in a series of well coordinated multi-pronged attacks, directed by Caliph Umar from Medina several thousand miles from the battlefields in Iran, becomes his greatest triumph, contributing to his reputation as a great military and political strategist.

By 651, most of the urban centers in Iranian lands, with the notable exception of the Caspian provinces and Transoxiana, have come under the domination of the Arab armies.

Many localities in Iran stage a defense against the invaders, but in the end none is able to repulse the invasion.

Even after the Arabs have subdued the country, many cities rise in rebellion, killing the Arab governor or attacking their garrisons, but reinforcements from the caliphs succeed in putting down all these rebellions and imposing the rule of Islam.

The violent subjugation of Bukhara after many uprisings is a case in point.

Conversion to Islam is, however, only gradual.

In the process, many acts of violence take place, Zoroastrian scriptures are burnt and many mobads executed.

Once conquered politically, the Persians begin to reassert themselves by maintaining Persian language and culture.

Regardless, Islam is adopted by many, for political, socio-cultural or spiritual reasons, or simply by persuasion, and becomes the dominant religion.