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.
Byzantine general and usurper Thomas the Slav continues his revolt against Emperor Michael II. He unsuccessfully besieges Constantinople, while his fleet is destroyed by Michael's fleet, using Greek fire.
King Ceolwulf I of Mercia invades Powys (Wales), but is beaten back by King Cyngen. However, Ceolwulf does destroy the fortress of Deganwy, and later takes the kingdom under his control (approximate date).
February 6 – Ukit Took becomes the last ruler (ajaw) of the Mayan city-state Copán (modern Guatemala). After his death in 830, the kingdom is wiped out, most likely from an epidemic.
Emperor Michael II defeats the rebel forces under Thomas the Slav in Thrace. He and his supporters are forced to seek refuge in Arkadiopolis (modern Turkey). After five months of blockade, Thomas surrenders and is delivered to Michael, seated on a donkey and bound in chains. He pleads for clemency and prostrates before Michael, but is executed.[7][8]
A group of Persio-Assyrian adherents of the Church of the East, under the leadership of two Persian bishops Prod (or Proth, also known as Aphroth) and Sappor (also known as Sabrisho), reach Kerala, India and reside in Quilon.
May – Euphemius, Byzantine admiral, organises an uprising in Sicily against Emperor Michael II. He proclaims himself Emperor (with the title of basileus) in Syracuse, independent from Constantinople. In turn, Euphemius is defeated by Byzantine troops (reinforcements from the East), and is driven out to North Africa.[11]
Æthelstan establishes himself as king of East Anglia, after killing King Ludeca of Mercia in battle. Ludeca is succeeded by Wiglaf, father-in-law (and probably distant cousin) of the late king Ceolwulf I's daughter.
Summer – Euphemius, Byzantine admiral, is murdered by emissaries from the Byzantine garrison at Castrogiovanni, which is besieged by the Muslims. Threatened by Byzantine reinforcements arriving from Constantinople, the survivors burn their ships and retreat overland westward to Mazara del Vallo.[16][17]
In the capital of Chang'an, a powerful court eunuch orders 50 wrestlers to arrest 300 commoners over a land property dispute in Northwest Chang'an, whereupon a riot breaks out in the streets.
Emperor Louis the Pious appoints his 6-year-old son Charles (by his second wife Judith) 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 (modern Norway). He expands his realm through military conquest and political negotiations, dividing the kingdom of Vestfold with his half-brother Olaf.
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.
^Lemerle, Paul (1965). "Thomas le Slave". Travaux et mémoires 1 (in French). Paris: Centre de recherche d'histoire et civilisation de Byzance. pp. 279–281, 291. OCLC457007063.