In the past Iran was a superpower that defeated the Roman Empire. Umar's army defeated Persia and made it Muslim by force. Had he not done so, would Iran, most probably, have remained a Zoroastrian nation today?
Islam does not force people to believe in it. No compulsion in religion. Majority of people in Iran embraced Islam willingly when they saw the great teachings of IIslam. Many Zoroastrians remained in their inherited faith till today. No one has forced any one to become a Muslim.
Wassalam.