The spread of Islam in Iran was not because of the sword. It was a reslut of the great morality of Islam. Regarding slavery, the Persian, The Romans, The Greeks, and most of Great nations practised it but in Islam whenm a slave embraces Islam he will beee free and this dioes not happen in other religions
Do you know about Ghadessiye war (happened between Iranians and Arabs in north of Kuwait in 1400 years ago) and Nahavand war (in nowadays Iran ) , they were war not morality exercise . With these two wars Arabs could come to Iran and make people Muslim not by speeches and teachings . If really the aim of Muslim was spreading Islam with peace they could send some Muslim groups to advertise Islam in Iran , like other religions . In those days there were many Christians , Jews , Mazdakian , Manavian and even Buddhists in Iran . Why Muslims sent army to Iran but not messengers ?
Did Christianity spread by wars or did it Buddhism ? In the world we have three international religions , Christianity , Islam and Buddhism , but it was only Islam that spread with wars and blood , so it is a shame for Islam and Arabs.
In Iran there weren’t any slaves before Islam , of course there were some Romans or Greek war prisoner but not any slave , Do you know Takht-e Jamshid is the first and only ancient memorial ( 2500 years old ) which not built by slaves but by workers . In 2500 years ago Cyrus the great freed all slaves and abandoned slavery in Iran but 1000 years later peaceful Muslims made thousands of Iranians slave’s of Arabs .

Regarding, worshipping Fire it is not accusation it is a real fact, it is not respect dued as you are saying, it is worshipping, prostrating in front of it. etc> which are clear signs of real worshipping.

Have you ever read Avesta (sanctified book of Zoroastrians ) or have you ever spoken to a Zoroastrian ? First read Avesta , speak to a Zoroastrian , then make decision . That was the same fault that your ancestors did about Zoroastrians .
AhuraMazda is the name of Zoroastrian God , Ahriman is the name of Devil , but Arabs thought that all the people of the world must say Allah to their Gods . Fire is the Symbol of clearness and brightness in Zoroastrian so Zoroastrians pay respect to it , even now in some rituals they make fire and pay respect to it , but fortunately in these days nobody comes from another country and kill them for doing this . Only Ahuramazda is being worshiped in Zoroastrians not fire , but because arabs didn’t know that they named Iranians as fireworshiper and killed them , what a wise God!

At that time, Iran was the second greatest Dynasty in the world. It was extremely advanced in science, weapons, number of soldiers than muslims. Can you tell me why did Islam defeat it and why those proud Iranians embraced Islam? They could have fled to other countries ?

At the time of Alexander , Iran was the greatest country in the world but Alexander with very smaller army could defeat Iran , At the time of Mongols , Monguls were very underdeveloped but they defeat Iran . Victories in the wars don’t prove that Islam is a good a peaceful religion . Was Alexander a Muslim ,or Mongols were Muslims ? .

They could have fled to other countries ? !!!!

What a great Idea !!
Do you want to say that now Iraqis love Americans and love Christianity that don’t flee from their own country .

The rise of Islam as a religion replacing Zoroastrianism is one of the greatest events in world history.

I don’t agree with you , I think it was the worst event that happened in all history , I think if arabs didn’t defeat us Ghadessiye war , now we’d have a better world , if Islam didn’t spread on the world and remained in Saudi Arabia , now we’d have a better world .