300 years after the rise of Islam there were Zoroastrians in Iran. The Muslim armies never forced people to accept Islam. It was only within Arabia that God ordered the idolaters to have a choice of either embracing Islam or fight against Muslims, because He wanted to remove this terrible idolatry that exited there. But outside Arabia where Islam met Christians, Jews, Zoroastrians and Hindus, they were given a choice by and large. That's why many Christians and Jewish communities survived in the Muslim world, but gradually many of them embraced Islam for different reasons.