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Year 900 (CM) was a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar.
- Spring – Atenulf I, Lombard prince of Capua, conquers the Duchy of Benevento. He deposes Duke Radelchis II and unites the two southern Lombard duchies in Mezzogiorno (Southern Italy). The Byzantines offer a strategic alliance to Atenulf who directs an campaign against the Saracens. They have establish themselves on the banks of the Garigliano River, from where Arab warbands launch frequent raids in Campania.
- February 4 – The 7-year-old Louis IV ("the Child") is at an assembly at Forchheim (Bavaria) proclaimed king of the East Frankish Kingdom. Because of his young age, the reins of government is entirely in the hands of others – the Frankish nobles and bishops. The most influential of Louis's councillors are Hatto I, archbishop of Mainz, and Solomon III, bishop of Constance.
- June 8 – Edward the Elder (son of Alfred the Great) is crowned king of England at Kingston upon Thames.
- June 17 – Baldwin II, Count of Flanders has Fulk the Venerable, bishop of Reims, assassinated.
- June 29 – The Venetians repel the Magyar raiders at Rialto.
- Summer – After the death of his wife Zoe Zaoutzaina, the Byzantine emperor Leo VI marries Eudokia Baïana.
- August – Abdallah, son of the Aghlabid emir Ibrahim II, represses a revolt of his Muslim subjects, and then initiates a campaign against the last Byzantine strongholds in Sicily.
- August 13 – Zwentibold, king of Lotharingia, is killed in battle on the Meuse River, while fighting against his rebellious subjects; subsequently they recognize Louis IV as their rightful suzerain.
- October 12 – Following Magyars raids in Lombardy, king Louis III ("the Blind") is called to Italy by the grandees. He takes Pavia, forcing king Berengar I to flee, and replaces him as King of Italy.
- King Donald II is killed after a 11-year reign. He is succeeded by his cousin Constantine II as king of Scotland; he will reign for more than 40 years.
- Docibilis I of Gaeta and his Saracen mercenaries attack Capua, in vain.
- After the rejection of their alliance proposal by the Bavarians, the Hungarians attack this country, occupying Pannonia and parts of Ostmark, which become part of the Hungarian state until today.
- Abū Ja'far al-Khāzin, Persian astronomer (d. 971)
- Adaldag, archbishop of Bremen (approximate date)
- Berengar II, king of Italy (approximate date)
- Berthold, duke of Bavaria (approximate date)
- Conrad, bishop of Constance (approximate date)
- Fujiwara no Saneyori, Japanese statesman (d. 970)
- Gero, archbishop of Cologne (approximate date)
- Gero, Frankish nobleman (approximate date)
- John of Gorze, Frankish abbot and diplomat (d. 974)
- Nicodemus of Mammola, Italian monk (d. 990)
- Ramiro II, king of León (approximate date)
- Ramwold, Frankish abbot (approximate date)
- Rasso, Frankish military leader (approximate date)
- Yang Pu, emperor of Wu (d. 939)
- June 17 – Fulk, archbishop of Reims
- August 13 – Zwentibold, king of Lotharingia (b. 870)
- Donald II, king of the Picts (Scotland)
- Dongshan Shouchu, Chinese Zen teacher
- Eardulf, bishop of Lindisfarne (approximate date)
- Fujiwara no Takafuji, Japanese nobleman (b. 838)
- Ibn Abi Asim, Muslim Sunni scholar (b. 822)
- John IX, pope of the Catholic Church
- Lde-dpal-hkhor-btsan, Indian ruler
- Litan, Irish abbot (approximate date)
- Liu Chongwang, chancellor of the Tang Dynasty
- Li Zhirou, chancellor of the Tang Dynasty
- Merfyn ap Rhodri, king of Powys (approximate date)
- Muhammad ibn Zayd, emir of Tabaristan (Iran)
- Ono no Komachi, Japanese poet (approximate date)
- Tadg mac Conchobair, king of Connacht (Ireland)
- Wang Tuan, chancellor of the Tang Dynasty
- Wulfhere, archbishop of York (approximate date)
- ^ Par Ṭabarī (translated by Franz Rosenthal) (1985). The return of the Caliphate to Baghdad. SUNY Press. ISBN 978-0-87395-876-9.
- ^ René Grousset (1885-1952) (1965) . L'empire des steppes, Attila, Gengis-Khan, Tamerlan (PDF) (4 ed.). Paris: Payot.
- ^ a b Louis Bréhier (1946). Vie et mort de Byzance (PDF). Paris: Albin Michel. p. 596.
- ^ Barbara M. Kreutz Before the Normans University of Pennsylvania Press, 1996 ISBN 978-0-8122-1587-8
- ^ Jacques Flach Les Origines de l'ancienne France. Volume 4 Ayer Publishing ISBN 978-0-8337-1147-2
- ^ N. J. Higham, David Hill Edward the Elder, 899-924 Routledge, 2001 ISBN 978-0-415-21497-1
- ^ Heinrich Joseph Wetzer Dictionnaire encyclopédique de la théologie catholique Gaume frères et J. Duprey, 1864
- ^ Enrico Guidoni La ville européenne: formation et signification du quatrième au onzième siècle Editions Mardaga, 1981 ISBN 978-2-87009-133-3
- ^ Theodora Antonopoulou The Homilies of the Emperor Leo VI BRILL, 1997 ISBN 978-90-04-10814-1
- ^ A. Charguéraud Les batards célèbres M. Lévy, 1859
- ^ Charles Albert Cingria La reine Berthe L'Age d'Homme, 1992 ISBN 978-2-8251-0347-0
- ^ Fiona Somerset Fry The history of Scotland Routledge, 1985 ISBN 978-0-415-06601-3
- ^ Caravale, Mario (ed). Dizionario Biografico degli Italiani XL Di Fausto – Donadoni. Rome, 1991.
- ^ N. Jayapalan (2001). History of India. Atlantic Publishers & Distri. ISBN 978-81-7156-928-1.
- ^ Artaud de Montor Histoire des souverains pontifes romains Didot, 1846