Al-Mamun vs John I Tzimiskes

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.

John I Tzimiskes became emperor after assassinating his uncle Nikephoros II Phokas. He was a skilled general and continued the military expansion of the Byzantine Empire.

John I led a campaign against the Rus under Sviatoslav I, who had invaded Bulgaria. He defeated the Rus at the Battle of Dorostolon in 971, forcing Sviatoslav to withdraw. This secured Byzantine control over the Balkans.

John I launched a major campaign into Syria, capturing Damascus, Beirut, and other cities. He advanced as far as Jerusalem, but did not capture it. These campaigns extended Byzantine influence in the Levant.

John I Tzimiskes died suddenly, possibly from poison. His death cut short his ambitious campaigns. He was succeeded by Basil II, who would later become known as the 'Bulgar-Slayer'.