Al-Mamun vs Orda Khan

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.

Orda Khan, as the eldest son of Jochi, inherited the eastern part of the Jochid ulus after his father's death in 1227. He established the White Horde, a semi-autonomous khanate within the Mongol Empire, ruling over the steppes of modern-day Kazakhstan.

Orda Khan led his forces as part of the Mongol invasion of Europe under Batu Khan. He commanded troops in the campaigns against the Rus' principalities, Poland, and Hungary, contributing to the Mongol conquest of Eastern Europe.

After the Mongol withdrawal from Europe in 1242, Orda Khan acknowledged the supremacy of his younger brother Batu Khan, who established the Golden Horde. Orda's White Horde remained a vassal, paying tribute and providing military support.