866

By place

Byzantine Empire

• April 21 - ; Bardas, the regent of the Byzantine Empire, is murdered by Basil the Macedonian at Miletus, while conducting a large-scale expedition against the Saracen stronghold of Crete.
• May 26 - Basil the Macedonian is crowned co-emperor of the Byzantine Empire, and is adopted by the much younger Michael III.

  Europe

• May 27 - King Ordoño I, ruler of the Kingdom of Asturias dies after a 16-year reign. He is succeeded by his son, Alfonso III, who later is referred to as "Alfonso the Great".
• July 2 - Battle of Brissarthe: Frankish forces, led by Robert the Strong, are defeated by a joint Breton-Viking army.
• Louis II, Holy Roman Emperor, defeats the Saracen invaders who are ravaging southern Italy.

  Britain

• The Great Heathen Army of the Vikings rides north to Northumbria. The Northumbrians are preoccupied with a civil war, and the Danes enter York unopposed.

  Abbasid Caliphate

• October 17 - Caliph al-Musta'in is put to death, after a 4-year reign. He is succeeded by al-Mu'tazz, who becomes the youngest Abbasid caliph to assume power.
• The Kharijite revolt against the Abbasid Caliphate begins in Al-Jazira, which will last for 30 years.

  Japan

• Fujiwara no Yoshifusa becomes regent to assist the child emperor Seiwa, starting the Fujiwara regency.

  By topic

Religion

• Boris I, ruler of the Bulgarian Empire, sends a diplomatic mission, led by the Bulgarian nobleman Peter, to Rome, in an effort to renew ties with the West.
• Pope Nicholas I forbids the use of torture, in prosecutions for witchcraft.