Why was Baha'u'llah imprisoned in Iran and why he went to Baghdad?

In August 1852, a small group of Babis in Tehran unsuccessfully attempted to assassinate the Shah—an event that was reported in several European newspapers and in diplomatic reports. Many Babis, including Baha’u’llah, were arrested, and about fifty of them were executed, some after cruel torture. Baha’u’llah, who clearly was not involved in the assassination attempt, avoided execution, however, and was instead put into a subterranean prison cell in Tehran. According to Baha’u’llah’s late writings, it was during this imprisonment in 1852 in Tehran that he had his first vision of his divine mission.

At the time of the assassination attempt on the Shah, Subh-i-Azal was involved in an abortive attempt to create a rebellion in his home province of Nur. Subh-i-Azal avoided arrest and succeeded in escaping from Iran to Baghdad. Many Babis chose to follow suit, left Iran and joined Subh-i-Azal in Baghdad, and before long, there was a sizeable Babi community in this city.

After four months of imprisonment, Baha’u’llah was released at the intervention of the Russian ambassador, whose legation secretary was Baha’u’llah’s brother in-law. Refusing an offer of exile in Russia, he went to Baghdad, and in January 1853, he joined the Babi exile community there.

(Citizens of the World - A History and Sociology of the Baha’is from a Globalisation Perspective by Margit Warburg)