Since its earliest days, members of the Baha'i' Faith have been labeled heretics and dissenters from the Iranian Islamic majority, despite being nonviolent (except in acts of self defense) and wholly apolitical. Claims that they serve as spies to foreign governments, stir dissent, or work to overthrow the Iranian government have been broadcast by the Islamic authorities since before the end of the Ottoman Empire. To many, the Baha'i' faith is nothing more than Islamaic Apostasy, leading to much of the violence towards them. As early as 1845, the Baha'i' predecessor religion (discussed below) had a Fatwa issued against its teachings.

To give a more detailed background, you have to start with the predecessor religion to the Baha'i' Faith, Babism. Sayyid Ali Muhammad Shirazi, also known as the Bab (meaning The Gate) was the founder of this new break from Islam, the Bab itself fulfilled the role of the Twelfth Imam (a messianic, prophetic figure, the role of which has been claimed by several leaders of religious movements including the Bab) in Twelver Shi'a Islam. Shirazi declared himself the Bab in 1844 and traveled around what was then Persia and Ottoman Iraq. His message of unity and peace, charisma, and his ability to successfully debate local and regional Islamic clergy gained his further popularity (and derision from Islamic Mullahs) as he traveled. What can be considered his most important message is that of the coming of the "Manifestation of God." A new prophet to lead the people into a new golden age and heaven on earth.

This spread of dissidence against the Islamic authorities angered the regional governments and they begin cracking down on adherents of this new Babism. The first recorded sign of violence is in 1847 when several Babis are killed in retaliation for the supposed murder of a Mullah who had declared the Bab a heretic. Following this, there is an escalation of action and reaction by Babis and the Persian government. Several more Babis are killed for various crimes and in late 1848, the Bab orders his top follower to lead the Babis to a shrine of a prominent Shi'a cleric and raise a black banner as a sign of unity among the Babis. What follows after is known as the Battle of Fort Tabarsi, in which between five and six hundred Babis (this should give some idea as to how the following had grown since 1844) built a makeshift fort and defended themselves from a Mullah's army for several months. They were defeated and several top Babis were killed. Two more incidents followed in the coming year, one at Zanjan, where a Persian army defeated and massacred several thousand Babis and the Nayriz Uprising in which much the same occurred after Babis murdered the local governor and were promptly executed by the Persian Military.

Both Zanjan and Nayriz were in progress (both taking several months) when the Bab was taken from the prison where he had been held, and publicly executed. After his death in January of 1850, there is little evidence of a decrease in Babi believers, it is just that the evidence isn't there, many of them had gone to ground and where they were found, they were mercilessly persecuted. By 1852, the Babi religion had a total number in the high tens of thousands (again, its difficult to give an exact number) and this high a number was a perfect springboard for the Baha'i' faith in which a Persian by the name of Mirza Hussein Ali Nuri, claimed to be the Manifestation of God that the Bab spoke of and he became Bahá'u'lláh (Glory of God).

When Bahá'u'lláh began proclaiming himself to be the next Manifestation, the Ottoman authorities banished him to Baghdad, then Constantinople, then Adrianople, and then finally in 1868, to his final stop of Akka (modern day Israel). Much of what became Baha'i' scripture such as The Book of Certitude and The Hidden Words were written in this time.

Sources: The Social Basis of the Babi Upheavals in Iran (1848-53): A Preliminary Analysis Moojan Momen,

Chronology of Persecutions of Babis and Baha'is

Bahá'u'lláh and the New Era J.E. Esslemont