Al-Mamun

Al-Mamun defeated his brother Al-Amin in a civil war over succession, culminating in the siege of Baghdad. Al-Amin was killed, and Al-Mamun became the sole caliph, but the conflict devastated the city and weakened the caliphate's authority.

Al-Mamun led multiple campaigns into Byzantine territory, capturing several fortresses including Loulon. These campaigns, though not decisive, secured the eastern frontier and demonstrated Abbasid military capability.

Al-Mamun established the Bayt al-Hikma (House of Wisdom) in Baghdad as an academy and library. It became the leading center for translation, research, and scholarship, attracting scholars from diverse backgrounds and advancing mathematics, astronomy, and medicine.

Al-Mamun ordered the construction of an observatory in the Shammasiyya district of Baghdad. Astronomers there made precise measurements of the solar year and planetary movements, improving the accuracy of astronomical tables and influencing later Islamic and European science.

Al-Mamun instituted the Mihna, a religious inquisition that required scholars to affirm the doctrine of the createdness of the Quran. This policy persecuted traditionalist scholars like Ahmad ibn Hanbal and created deep divisions within Islamic society.