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]
February 11 – Pope Paschal I, leader of the Roman Catholic Church and ruler of the Papal States,dies after a reign of seven years.[10] The Roman Curia refuses to bury him in St. Peter's Basilica because of his harsh treatment of the people of Rome, and inters him instead in the Basilica of Santa Prassede.[11] The office remains vacant for four months before a new conclave is assembled.
March 3 &ndash (9.19.13.15.19) (9th B'ak'tun, 19th K'atun, 13th year (Tun) 15th month (Winal) 19th day (K'in)) Juun Tsak-Took becomes the new ruler of the Mayan city state of Machaquila (in what is now Guatemala), along with co-ruler and Ti-Chaak become the after the death of Sihyaj K'in Ich’aak II. Juun Tsak-Tookwill until his 840.
May – In Constantinople and throughout the Byzantine Empire, Michael II celebrates the festival of Triumph to mark his return to the capital after defeating the rebellion of Thomas the Slav[15][16]
June 4 – Pope Eugene II is selected by church leaders and the local nobility of the Papal States as the 99th Pontiff of the Roman Catholic Church, succeeding Pope Paschal I, receiving a majority after the more conservative Zinzinnus is rejected.[17]
September – Rebellious Arab troops in North Africa, led by Mansur ibn Nasr al-Tunbudhi in a battle against the Aghlabids, led by the Emir Ziyadat Allah, invade and occupy the city of Tunis.[18]
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.[22]
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.[27][28]
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.
^Kennedy, Hugh (2018). "The Origins of the Aghlabids". In Anderson, Glaire D.; Fenwick, Corisande; Rosser-Owen, Mariam (eds.). The Aghlabids and Their Neighbors: Art and Material Culture in Ninth-Century North Africa. Brill. p. 47. ISBN978-90-04-35566-8.
^Marios Costambeys, Power and Patronage in Early Medieval Italy (Cambridge: 2007), 342–343.