The Muslim conquest of Persia led 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 Persia. The Sassanid Empire was first invaded by Arabs in present day Iraq in 633 under general Khalid ibn Walid resulting in the Muslim conquest of Iraq.
Following the transfer of Khalid to the Roman front in the Levant, the Muslims eventually lost Iraq to Persian counterattacks. The second invasion of Iraq began in 636 under Saad ibn Abi Waqqas when a key victory at the Battle of Qadisiyyah led to the permanent end of Sassanid control west of Persia. The Zagros mountains then became a natural barrier and border between the Rashidun Caliphate and the Sassanid empire.
Owing to continuous raids by Persians (who were threatened by the Arabs) into Iraq, Caliph Umar ordered a full invasion of the Sassanid Persian empire in 642 which was completed with the complete conquest of the Sassanids by mid 644. The quick conquest of Persia in a series of well coordinated multi-pronged attacks, directed by Caliph Umar from Madinah several thousand miles from the battlefields in Persia, became his greatest triumph, contributing to his reputation as a great military and political strategist.
Zoroastrians, as well as many other minority religious communities in Iran are still persecuted today. KGS