Otto I of Brunswick-Lüneburg

The current Duke of Brunswick, called Otto the Child to distinguish him from his uncle, Holy Roman Emperor Otto IV, had achieved an agreement with the Hohenstaufen Emperor Frederick II that had ended the dispute between the Hohenstaufen House and the House of Welf, to which Otto belongs.

This dispute had culminated when Henry the Lion had been stripped of his duchies in 1180 by Frederick's grandfather, Frederick Barbarossa.

According to this agreement, Otto had transferred all of his private property to the Emperor, who immediately returned it to him as a hereditary imperial fief.

Otto had in this way reacquired the status, lost by Henry the Lion, of a prince of the Holy Roman Empire.

The Duchy of Saxony, which Henry had held, had since passed on to Ascanian dukes, so that the Emperor had had to create for Otto a new duchy: Brunswick-Lüneburg, named after the two central cities around which Otto's former properties are located.

Otto can substantially increase his territory by supporting King William, who marries his daughter Elizabeth in January of 1252.

William had intimated to the princes of Germany his desire to meet them in a general diet at Frankfurt against the Feast of St. John the Baptist; he is preparing to leave Brunswick with his father-in-law for the purpose of being present at this assembly when Otto is suddenly taken ill and dies on June 9.

Otto, buried in Brunswick Cathedral, is the male-line ancestor of all later members of the House of Welf.

Albert, the oldest surviving son, takes over the rule of the duchy; his younger brother John will later join him.