He went into exile for several years, but was then allowed to stay on the (allodial) estates inherited from his mother's side until the end of his life.

At the Imperial Diet of 1235 in Mainz, as part of the reconciliation between the Hohenstaufen and Welf families, Henry's grandson, Otto the Child, transferred his estates to Emperor Frederick II and was enfeoffed in return with the newly created Duchy of Brunswick-Lüneburg, which was formed from the estates transferred to the Emperor as well as other large areas of the imperial fisc.

In 1269, the duchy was divided,[1][2] Albert receiving the southern part of the state around Brunswick and John the northern territories in the area of Lüneburg.

The various parts of the duchy were further divided and re-united over the centuries, all of them being ruled by the Welf or Guelph dynasty, who maintained close relations with one another—not infrequently by marrying cousins—a practice far more common than is the case today, even among the peasantry of the Holy Roman Empire, for the contemporary salic inheritance laws encouraged the practice of retaining control of lands and benefits.

The seats of power moved in the meantime from Brunswick and Lüneburg to Celle and Wolfenbüttel as the towns asserted their independence.

To the north, this new state bordered on the County of Hoya near Nienburg and extended from there in a narrow, winding strip southwards up the River Leine through Wunstorf and Hanover where it reached the Principality of Wolfenbüttel.

The state lay along the northern part of the Solling hills and the River Leine near Einbeck and north of the Eichsfeld on and in the southwestern Harz.

Other branches that did not have full sovereignty included the states of Dannenberg, Harburg, Gifhorn, Bevern, Osterode, Herzberg, Salzderhelden, and Einbeck.

While a total of about a dozen subdivisions that existed, some were only dynastic and not recognised as states of the Empire, which at one time had over 1500 such legally recognized entities.

One of the dynastic lines was that of the princes of Lüneburg, which, in 1635, acquired Calenberg for George, a junior member of the family who set up residence in the city of Hanover.

His son Christian Louis, and his brothers inherited Celle in 1648 and thereafter shared it and Calenberg between themselves; a closely related branch of the family ruled separately in Wolfenbüttel.

In this event, George I succeeded his second cousin Anne, Queen of Great Britain—the last reigning member of the House of Stuart—and subsequently formed a personal union on August 1st, 1714 between the British crown and the duchy of Brunswick-Lüneburg (electorate of Hanover), which would last until well after the end of the Napoleonic wars more than a century later—including even through the dissolution of the Holy Roman Empire and the rise of a new successor kingdom.

George III contested the validity of the dissolution of the Empire and maintained separate consular offices and staff for the Electorate of Hanover until the Congress of Vienna at the war's end.

When the Wolfenbüttel Line became extinct in 1884, the German government withheld the rightful heir, the Crown Prince of Hanover, from taking control, instead installing a regency.

However, 1373–1388 would be the only period in which a Brunswick-Luneburg land was not ruled by a Welf: In the wake of his death, Elector Wenceslas appointed Bernard, his brother-in-law, as co-regent involved him in the government.

But his younger brother Henry did not agree with this ruling, and after vain attempts to reach an agreement, the fight flared up again in the spring of 1388.

Lüneburg continued the preparations, formed an alliance with the Prince-Bishop of Minden and Count of Schaumburg and set up his own army.