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Who was Otto IV and how was he allied with King John?

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John John of England, from an early 14th-century illumination.

Otto IV (c. 1175–1218) was a member of the Welf dynasty who served as a German king and Holy Roman emperor. He was the son of Henry the Lion and Matilda, daughter of Henry II of England, making him the nephew of King John.12

Otto’s alliance with King John was rooted in both family ties and shared political interests. Having been raised at the court of his uncle Richard I, Otto received significant English financial and diplomatic support during his struggles against the Hohenstaufen faction.1

In 1214, John organized an international coalition to reclaim English lands lost to the French king Philip II. Otto joined this alliance, intending to invade France from the north while John attacked from the west. However, the coalition suffered a disastrous defeat at the Battle of Bouvines on July 27, 1214. Following this failure, Otto was deserted by his supporters and deposed in 1215, while John’s weakened position forced him to sign the Magna Carta.21