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Year 865 (DCCCLXV) was a common year starting on Monday of the Julian calendar.

• King Louis the German divides the East Frankish Kingdom among his three sons. Carloman receives Bavaria (with more lands along the Inn River). He gives Saxony to Louis the Younger (with Franconia, and Thuringia) and Swabia (with Raetia) to Charles the Fat. Louis arranges marriages into the local aristocracy, for his sons to hold important territories along the frontiers.
• King Lothair II, threatened with excommunication, takes back his first wife, Teutberga. She expresses her desire for an annulment, but this is refused by Pope Nicholas I.
• Boris I, ruler (knyaz) of the Bulgarian Empire, suppresses a revolt, and orders the execution of 52 leading boyars, along with their whole families.

• The Great Heathen Army (probably no more than 1,000 men) of Vikings, led by Ivar the Boneless and Halfdan Ragnarsson, invades East Anglia. King Edmund of East Anglia buys peace with a supply of horses.
• Viking king Ragnar Lodbrok is captured by the Northumbrians in battle, and killed by being thrown into a pit filled with venomous snakes, on the orders of King Ælla of Northumbria.
• Autumn – King Æthelberht of Wessex dies after a 5-year reign, and is buried at Sherborne Abbey (Dorset). He is succeeded by his brother Æthelred I, as ruler of Wessex.

• Caliphal Civil War: An armed conflict starts between the rival Muslim caliphs al-Musta'in and al-Mu'tazz. They fight to determine who takes control over the Abbasid Caliphate (until 866).

• Kassia, a Byzantine abbess and hymnographer, dies. She is one of the first Early Medieval composers of many hymns.

• Al-Nayrizi, Persian mathematician (d. 922)
• Baldwin II, Frankish margrave (approximate date)
• Jinseong, queen of Silla (approximate date)
• Lady Ren Neiming, Chinese noblewoman (d. 918)
• Louis III, king of the West Frankish Kingdom (or 863)
• Simeon I, ruler (khan) of the Bulgarian Empire (or 864)

• February 3 – Ansgar, Frankish monk and archbishop (b. 801)
• March 8 – Rudolf of Fulda, German theologian
• November 11 – Petronas, Byzantine general
• December 26 – Zheng, empress of the Tang dynasty
• Æthelberht, king of Wessex^[1]
• Antony the Younger, Byzantine governor and saint (b. 785)
• Deshan Xuanjian, Chinese Zen Buddhist monk
• Gao Qu, chancellor of the Tang dynasty
• Kassia, Byzantine abbess and hymnographer
• Khurshid, ruler (shah) of Daylam
• Liu Gongquan, Chinese calligrapher (b. 778)
• Lothair the Lame, Frankish abbot
• Muhammad ibn al-Fadl al-Jarjara'i, Muslim vizier (or 864)
• Pepin II, king of Aquitaine (approximate date)
• Ragnar Lodbrok, king of Denmark and Sweden
• Raymond I, count of Toulouse
• Rorgon II, count of Maine (approximate date)
• Tigernach mac Fócartai, king of Lagore (Ireland)
• Wenilo, Frankish archbishop
• Xiao Zhi, chancellor of the Tang dynasty
• Yahya ibn Umar, Muslim imam (or 864)