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.

Italy

• An army in Lucera is assembled by orders of Louis II in preparation for an attack on the Emirate of Bari.

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 (Upper Mesopotamia), 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 First [Bulgarian Empire|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 orders that all Catholics should abstain from eating the "flesh, blood, or marrow" of warm-blooded animals on Wednesdays and Fridays.
• Pope Nicholas I forbids the use of torture in prosecutions for witchcraft.

Births

• June 10 - Uda, emperor of Japan
• September 19 - Leo VI, Byzantine emperor
• Carloman II, king of the West Frankish Kingdom
• Robert I, king of the West Frankish Kingdom
• Yao Yi, chancellor of Later Tang

Deaths

• April 21 - Bardas, Byzantine chief minister and regent
• May 27 - Ordoño I, king of Asturias
• June 21 - Rodulf, Frankish archbishop
• July 2 - Robert the Strong, Frankish nobleman
• July 16 - Irmgard, Frankish abbess
• October 17 - Al-Musta'in, Abbasid caliph
• Adelaide of Tours, Frankish noblewoman
• Al-Mu'ayyad, Abbasid prince
• Charles the Child, king of Aquitaine
• Eberhard, duke of Friuli
• Emenon, Frankish nobleman
• Hungerus Frisus, bishop of Utrecht
• Linji Yixuan, Chinese monk and founder of the Linji school
• Liudolf, duke of Saxony
• Ranulf I, Frankish nobleman
• Robert, Frankish nobleman
• Rudolph, Frankish nobleman
• Wang Shaoyi, general of the Tang Dynasty
• Yahya ibn Yahya, Idrisid emir of Morocco