821

By place

Byzantine Empire

• Byzantine general Thomas the Slav leads a revolt and secures control over most of the Byzantine themes in Anatolia. He gets recognition from the Abbasid Caliphate, and concludes a peace treaty with Caliph al-Ma'mun. Thomas crosses with his fleet from Abydos to Thrace, and blockades Emperor Michael II in Constantinople; but Thomas' first attack on the capital fails.

  Europe

• February - Duke Borna of Croatia dies after an 11-year reign, as vassal of the Frankish Empire. He is succeeded by his nephew, Vladislav. Emperor Louis I recognizes him as prince of Dalmatia and Liburnia, at the Council of Aachen.
• October - Lothair I, co-emperor and eldest son of Louis I, marries Ermengarde in Thionville. She is the daughter of Count Hugh of Tours.

  Britain

• King Coenwulf of Mercia dies in Basingwerk near Holywell, while preparing for another assault on Powys, and is buried in Winchcombe Abbey. He is briefly succeeded by his son Cynehelm, but he is killed, probably fighting the Welsh, though supposedly through the treachery of his sister Cwenthryth. The Mercian throne passes to Coenwulf's brother, Ceolwulf I.

  Abbasid Caliphate

• Tahir ibn Husayn, an Iranian general, is appointed to govern Khurasan, as a reward for supporting the Abbasid caliph al-Ma'mun in the Fourth Fitna. This begins the rule of the Tahirid dynasty over Khurasan, which will last until 873.