How did Islam enter Iran, and what is the history of its Islamic conquest?

The Islamic conquest of Iran—which suffered from injustice and corruption under the Sassanids and Zoroastrianism—began during the reign of Abu Bakr Al-Siddiq, may Allah be pleased with him, with Al-Muthanna ibn Haritha conquering some lands in 13 AH.

During the reign of Omar, may Allah be pleased with him, Abu Ubaid al-Thaqafi led the invasion of Iran and was victorious in the Battle of the Bridge, but he was later killed.

Then, Sa'd ibn Abi Waqqas, may Allah be pleased with him, completed the conquest in 14 AH at the Battle of Qadisiyyah, where the Persians suffered a crushing defeat and their leader Rustam was killed.

The Muslims continued their advance, victorious in the Battle of Jalula in 18 AH, then in the decisive Battle of Nihawand in 21 AH, which was called "the Conquest of Conquests," after which the Sassanids never rose again.

The Muslim control over Iran lasted for about ten years, and the migration of Arab tribes contributed to the spread of Islam. Iran remained Sunni for about nine centuries until the Shiite Safavid state took control of it in 906 AH.