The Mihna began as a Mu'tazilite inquisition which lasted from 833-848 A.D. Between 813-827, the Abbasid Caliph Al-Ma'mum sponsored the translation of Greek philosophy and science into Arabic. Al-Ma'mum was a Mu'talizite - one who believed that Islam could be proven through reason. In 827 he had grown convinced that the Qu'ran was divinely created, and set up the Mihna, the so-called "Muslim Inquisition". The Mihna led to the imprisonment and/or exile of popular figures like Ibn Hamal, Al-Rawandi, and Abu Tammam.
The inquisition began with the permission of Caliph al-Ma'mun who supported the Mu'tazilite faction. The Mihna was a test for the Qadis of Baghdad. They were questioned concerning their opinions on the creation of the Qur'an. The Qadis who passed the test went out to test others. There was little interest in Damascus, among the Qadis, for such a test, so the Caliph had to perform the test.
The Mihna continued into the Caliphate of al-Mutawakkil who, in turn, persecuted the Mu'talizites. The number of Qadis supporting Mu'tazilism, declined, and some were imprisoned. By then, the idea that the Qur'an had been created was banned, and violations on this ban could result in death.