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Events

October 2 - Emperor Michael II dies after an 8-year reign in Constantinople, and is succeeded by his 16-year-old son Theophilos, as sole emperor of the Byzantine Empire. He continues his father's ideology of iconoclasm.
October - Battle of Thasos: Saracens from the newly founded Emirate of Crete almost annihilate the Byzantine fleet at Thasos, close to the coast of Thrace. The Cyclades and other islands in the Aegean Sea are pillaged.
Europe
Emperor Louis the Pious appoints his 6-year-old son Charles as ruler of the Frankish subkingdom Alamannia, enraging his eldest son and co-emperor Lothair I, who begins an insurrection.
Viking chieftain Halfdan the Black becomes king of Agder. He expands his realm through military conquest and political negotiations, dividing the kingdom of Vestfold with his half-brother Olaf.
Giustiniano Participazio, doge of Venice, dies after a 2-year reign, and is succeeded by his younger brother Giovanni Participazio. He continues the work of Giustiniano, in construction of St. Mark's Basilica.
Britain
King Egbert of Wessex invades Mercia, ousts his rival Wiglaf, and attempts to rule directly from Wessex. He is recognized as overlord of other English kingdoms.
Winter - Battle of the River Dore: Egbert of Wessex leads his army against the Northumbrians as far as Dore, where he clashes with King Eanred of Northumbria.
Egypt
The Nile River freezes over.
China
The Bai kingdom of Nanzhao captures the city of Chengdu, in Sichuan Province.
By topic

Ansgar, Frankish abbot of Corvey, is appointed missionary to Sweden by Louis the Pious, at the request of the Swedish king Björn at Haugi.
The city of Wiesbaden is first mentioned by Einhard, biographer of former emperor Charlemagne.