821

Events

Byzantine general Thomas the Slav leads a revolt, and secures control over most of the Byzantine themes (provinces) 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.

February – Duke Borna of Croatia dies after an 11-year reign, as vassal of the Frankish Empire. He is succeeded by his nephew, Vladislav.<ref name="Mladjov">Template:Cite web Template:Dead link</ref> 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 (northeastern France). She is the daughter of Count Hugh of Tours.

King Coenwulf of Mercia dies in Basingwerk near Holywell (Wales), 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.

By the time Al-Ma'mun became caliph, the Arabs and the Byzantines had settled down into border skirmishing, with Arab raids deep into Anatolia to capture booty.
Tahir ibn Husayn, an Iranian general, is appointed as governor of Khurasan, as a reward for supporting the Abbasid caliph al-Ma'mun in the Fourth Fitna.
Caliph Al-Ma'mun appointed Nu'aym ibn al-Waddah al-Azdi as the governor of Yemen in 821.