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Events

September 24 – Emperor Michael III is murdered, by order of his co-emperor Basil I. Basil becomes sole ruler (basileus) of the Byzantine Empire, and founds the Macedonian Dynasty (until 1056). Basil rebuilds the Byzantine army and navy, in an effort to restore the empire.{{cite book |last=Finlay |first= G. |year=1856 |title= History of the Byzantine Empire from DCCXVI to MLVII |edition= 2nd |publisher= W. Blackwood | pages= 180–181}}

August – Treaty of Compiègne: King Charles the Bald cedes the Cotentin Peninsula to Salomon, duke ('king') of Brittany, after he had sent his son-in-law Pascweten to negotiate a peace. Charles orders the fortification of the cities of Tours, Le Mans and Compiègne.
Bořivoj I declares himself duke (knyaz) of Bohemia, and founds the Přemyslid Dynasty (approximate date).

Vikings or "Danes" (the two terms were often used interchangeably at the time), comprising the Great Heathen Army, advance northward from bases in the Kingdom of East Anglia, into the Anglo-Saxon Kingdom of Northumbria.
Deira, the southernmost part of Northumbria, is conquered by the Vikings. Ivar the Boneless, one of their leaders, installs a puppet king of Northumbria, Ecgberht I.Paul Hill (2009). The Viking Wars of Alfred the Great, p. 30. {{ISBN|978-1-59416-087-5}}.
The rival monarchs of Northumbria, Ælla and Osberht, join forces in an attempt to expel the Great Heathen Army, but are defeated in battle by Ivar the Boneless and Halfdan Ragnarsson. Osberht is killed in battle, while Ælla is reportedly captured, before being subject to the blood eagle: a combined method of torture and execution.
Surviving members of the Northumbrian court flee into the northernmost part of the kingdom, Bernicia.

The Council of Constantinople is held (presided over by Patriarch Photius), which anathematizes the use of the Filioque clause in the Creed, and also Pope Nicholas I, for his attacks on the work of Greek missionaries in Bulgaria.
September – Photius I ("the Great"), patriarch of Constantinople, is removed from office and banished. Ignatius is reinstated as patriarch by Basil I.
November 13 – Pope Nicholas I dies after a 9-year reign. He is succeeded by Adrian II (also referred to as Hadrian II), as the 106th pope of Rome.