861

By place

Europe

• March - Robert the Strong is appointed margrave of Neustria by King Charles the Bald. He re-establishes the Breton March, and extends his remit by campaigning against Salomon, duke 'king' of Brittany. Robert hires a combined Seine-Loire fleet for 6,000 pounds of silver, 'before Salomon can ally with them against him'. In return, Salomon enlists 12 Viking ships under the command of Hastein, to raid the county of Maine, which, with Anjou, becomes squeezed between Brittany and Neustria.
• Spring - Council of Constantinople, attended by 318 fathers and presided over by papal legates confirms Photius the Great as patriarch and passes 17 canons.
• Carloman, eldest son of King Louis the German, revolts against his father. He is captured, but manages to escape to the Ostmark.
• Summer - Viking raiders sack the cities of Paris, Cologne, Aachen, Worms and Toulouse.

  Abbasid Caliphate

• December 11 - Caliph al-Mutawakkil is murdered by his Turkish guard, starting the period of troubles known as the "Anarchy at Samarra". He is succeeded by his son Al-Muntasir, as ruler of the Abbasid Caliphate.
• Ya'qub ibn al-Layth, a Muslim military leader, founds the Saffarid Dynasty. He rules over parts of Khurasan and eastern Iran, and establishes his capital at Zaranj.

  By topic

Hydrology

• Al-Mutawakkil orders the construction of a Nilometer on Rhoda Island in central Cairo, supervised by the Persian astronomer Ahmad ibn Muhammad ibn Kathir al-Farghani.