What was the Fitna of Al-Ma'mun, and what happened to Ibn Hanbal during it?

Al-Ma'mun, one of the Abbasid caliphs and son of Harun al-Rashid, assumed the caliphate in 198 AH. He was influenced by the Mu'tazilah, who embellished for him the doctrine of the creation of the Qur'an and the negation of God's attributes. Caliphs before him held the creed of the Salaf, but he inclined towards the opinion of the Mu'tazilah. During his expedition to conquer the Romans in Tarsus, he ordered his deputy in Baghdad to invite people to the doctrine of the creation of the Qur'an and threatened the imams of hadith. Some of them complied under duress. Imam Ahmad ibn Hanbal and Muhammad ibn Nuh refused, so they were carried to Al-Ma'mun, who died before Imam Ahmad reached him, after Imam Ahmad's supplication.

He was succeeded by his brother Al-Mu'tasim, who continued to uphold the same doctrine. He ordered Imam Ahmad to be beaten and imprisoned. The ordeal continued until Al-Mutawakkil assumed the caliphate. He loved the Sunnah, lifted the ordeal, prohibited discussion on the doctrine of the creation of the Qur'an, and honored Imam Ahmad, who would not accept the caliph's gifts except to distribute them among the poor.