|Covid-19|August 8 – Treaty of Meerssen: King Louis the German forces his half-brother Charles the Bald to accept a peace treaty, which partitions the Middle Frankish Kingdom into two larger east and west divisions. Louis receives most of Austrasia (which evolves into the Kingdom of Germany), and Charles receives territory in Lower Burgundy (which evolves into the Kingdom of France). However, large parts of the Frisian coast are under Viking control.
Charles the Bald marries Richilde of Provence, after the death of his first wife, Ermentrude of Orleans. He intends to secure his rule in Lotharingia through the powerful Bosonid family and the connection to Teutberga, widow-queen of Lothair II.
Rastislav, ruler (knyaz) of Great Moravia, dies in prison after he is condemned to death for treason, by Louis the German. He is succeeded by his nephew Svatopluk I, who becomes a vassal of the East Frankish Kingdom.
Bořivoj I, duke of Bohemia, makes Levý Hradec (modern Czech Republic) his residence. Around this time Prague Castle is founded (approximate date).
Wilfred the Hairy, a Frankish nobleman, becomes count of Urgell and Cerdanya (modern-day Catalonia).
Northern Autumn – The Great Heathen Army, led by Ivar the Boneless and Ubba, invades East Anglia and plunders Peterborough. King Edmund the Martyr is captured, tortured, beaten and used as archery practice (or 869).
The Danes, led by Ivar the Boneless and King Olaf of the Dublin Vikings, besiege Dumbarton in Scotland, the capital of King Artgal of Stratchlyde. After a siege of four months, the citadel is captured and destroyed.
The Danes, led by Halfdan Ragnarsson and Bagsecg, invade Wessex and take the royal estate at Reading (Berkshire), which Halfdan makes his headquarters. A naval Viking contingent sails up the Thames River.
December 31 – Battle of Englefield: The Vikings clash with ealdorman Æthelwulf of Berkshire. The invaders are driven back to Reading; many of the Danes (including one of the earls named Sidrac) are killed.
Byzantine–Arab War: An Muslim expeditionary force, led by Halaf al-Hadim, Arab governor of Sicily, conquers Malta. He is welcomed by the local Christian inhabitants as liberator of the agonizing Byzantine yoke. The Muslim invaders loot and pillage the island, destroying the most important buildings.
June 21 – Caliph Al-Muhtadi is deposed and killed by the Turks, after a brief reign. He is succeeded by Al-Mu'tamid (son of the late Al-Mutawakkil) as ruler of the Abbasid Caliphate, moving his court to Baghdad. End of the "Anarchy at Samarra".
The Zanj Rebellion: The Zanj (black slaves from East Africa) capture the Abbasid seaport of Al-Ubdullah at the Persian Gulf, and cut off communications with Basra (modern Iraq).
February 28 – The Fourth Council of Constantinople ends. The Bulgarians are granted an autonomous archbishopric. with its seat in the capital of Pliska.
Æthelflæd, female ruler of Mercia (d. 918)
Alexander III, Byzantine emperor (d. 913)
Bernard, illegitimate son of Charles the Fat (d. 891)
Ebalus, duke of Aquitaine (approximate date)
Fulk I, Frankish nobleman (approximate date)
Lde-dpal-hkhor-btsan, Indian ruler (approximate date)
Petar, prince of Serbia (approximate date)
Romanos I, Byzantine emperor (d. 948)
Theodora, Roman politician (approximate date)
Wang Dingbao, Chinese chancellor (d. 941)
Zhu Yanshou, Chinese governor (d. 903)
Zwentibold, king of Lotharingia (d. 900)
January 29 – Salih ibn Wasif, Muslim general
February 4 – Ceolnoth, archbishop of Canterbury
April 2 – Æbbe the Younger, Frankish abbess
June 21 – Al-Muhtadi, Muslim caliph
September 1 – Muhammad al-Bukhari, Persian scholar (b. 810)
November 20 – Edmund the Martyr, king of East Anglia (or 869)
December 4 – Suairlech ind Eidnén mac Ciaráin, Irish bishop
December 27 – Aeneas of Paris, Frankish bishop
Adarnase II, Georgian Bagratid prince (approximate date)
Al-Harith ibn Sima al-Sharabi, Muslim governor
Al-Zubayr ibn Bakkar, Muslim historian (b. 788)
Caesar of Naples ("the Brave"), Italian admiral
Gregory III, co-regent and duke of Naples
He Quanhao, general of the Tang Dynasty (b. 839)
Judith of Flanders, queen of Wessex and countess of Flanders (b. 843)
Muhammad al-Bukhari, Persian Islamic scholar (b. 810)
Neot, English monk and saint (approximate date)
Rastislav, ruler (knyaz) of Great Moravia
Ratramnus, Frankish monk and abbot (approximate date)
Wen Tingyun, Chinese poet and lyricist (b. 812)
Year 870 (DCCCLXX) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar.