1203Centuries: 12th century - 13th century - 14th century
- April 16 - Philip II of France enters Rouen, leading to the eventual unification of Normandy and France.
- June 23 - Fleet of the Fourth Crusade enters Bosphorus
- July 17 - Fourth Crusade captures Constantinople by assault; the Byzantine emperor Alexius III flees from his capital into exile.
- August 1 - Fourth Crusade elevates Alexius IV as Byzantine emperor, after the citizens of Constantinople proclaim as emperor Isaac II Angelus (Alexius IV's father).
- William de Braose, Fourth Lord of Bramber becomes the guardian of Arthur I, Duke of Brittany, and is possibly responsible for his death.
- Minamoto no Sanetomo becomes shogun of the Kamakura Shogunate