813

By place

Byzantine Empire

• June 22 - Battle of Versinikia: The Bulgars, led by Krum, ruler of the Bulgarian Empire, defeat Emperor Michael I near Edirne. The Byzantine army is destroyed by a counter-attack of Bulgarian heavy cavalry, while trapped in the valley. Krum captures the Byzantine camp and a rich prize, including gold and weaponry.
• July 11 - Michael I, under threat by conspiracies, abdicates in favor of his general Leo the Armenian, and becomes a monk. His sons are castrated to prevent them succeeding the Byzantine throne, and relegated into monasteries. One of them, Niketas, eventually becomes a patriarch of Constantinople.
• July 17 - Krum reaches Constantinople, and sets his camp outside the walls. He is given an invitation, and a promise of safe conduct, to meet Leo V. Krum sets out unarmed for the capital with only a small escort, but is ambushed and manages to escape. After this unsuccessful Byzantine murder attempt, the Bulgars ravage much of Eastern Thrace.
• Autumn - Siege of Adrianople: Krum captures Adrianople—one of the most important Byzantine fortresses in Thrace—after being attacked with siege engines. The garrison is forced to surrender, due to starvation. On the orders of Krum, the population of the surrounding area is transferred to Bulgarian territory, north of the Danube.
• Ashot I becomes the first Georgian Bagratid prince of Iberia, under Byzantine protection.

  Europe

• September 11 — Louis the Pious, king of Aquitaine, is crowned co-emperor of the Franks, with his father Charlemagne.
• Danish Viking raiders, led by King Horik I, attack Vestfold, due to its insubordination.
• Third Council of Tours: Priests are ordered to preach in the vernacular language.

  Abbasid Caliphate

• Autumn - Siege of Baghdad: Caliph al-Amin surrenders Baghdad, after al-Ma'mun's General Tahir accepts his peace terms, but he is captured and executed. His brother al-Ma'mun becomes undisputed ruler of the Abbasid Caliphate.

• The Baghdad School of Astronomy is opened by al-Ma'mun.
• Caliph al Ma’mun founds a school in Baghdad called the House of Wisdom. In this school scholars translated Greek philosophy classics into Arabic.