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Year 1204 (MCCIV) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar.
Conquest of Constantinople by the Crusaders
- April 13 – Fourth Crusade: The Crusaders take Constantinople by storm, and pillage the city for 3 days. Forces of the Republic of Venice seize the antique statues that will become the horses of Saint Mark.
- May 16 – Baldwin, Count of Flanders is crowned emperor of the Latin Empire a week after his election, by the members of the Fourth Crusade.
- Theodore I Laskaris flees to Nicaea after the capture of Constantinople, and establishes the Empire of Nicaea; Byzantine successor states are also established in Epirus and Trebizond.
- Boniface I, Marquess of Montferrat, a leader of the Fourth Crusade, founds the Kingdom of Thessalonica.
- The writings of French theologian Amalric of Bena are condemned by the University of Paris, and Pope Innocent III.
- Tsar Kaloyan is recognized as king of Bulgaria by Pope Innocent III, after the creation of the Bulgarian Uniate church.
- Valdemar II of Denmark is recognized as king in Norway.
- Angers and Normandy are captured by Philip II of France.
- The Cistercian convent of Port-Royal-des-Champs is established.
- The district of Cham becomes subject to Bavaria.
- Hermann I, Landgrave of Thuringia submits to Philip of Swabia.
- Beaulieu Abbey is founded.
- The Channel Islands of Guernsey and Jersey decide, after a plebiscite of wealthy land owners, to remain with the English crown, after Normandy is recaptured by Philip II of France.
- January 1 – King Haakon III of Norway
- January – Isaac II Angelos, Byzantine emperor
- February 8 – Alexios IV Angelos, Byzantine emperor
- April 1 – Eleanor of Aquitaine, Sovereign Duchess Regnant of Aquitaine, queen of France and England
- August 11 – King Guttorm of Norway
- August 14 – Minamoto no Yoriie, Japanese shogun (b. 1182)
- September 30 or November 30 – Emeric, King of Hungary (b. 1174)
- November 30 – Emeric, King of Hungary
- November –Ban Kulin, ruler of Bosnia (b. 1163)
- December 12 (or December 13) – Maimonides, Spanish rabbi and philosopher (b. 1135)
- date unknown
- probable – Amalric of Bena, French theologian