The state emerged from the inheritance of the first Saxon state of Henry the Lion. Shortly after Henry's death the duchy of Saxony began to decline. Several mini-states came into being, among them Brunswick and Lüneburg.
The first duke of Brunswick-Lüneburg was Otto the child, who reigned from 1235 on. After 1267 the duchy was split into two partial states (which later became a multitude of partial states), but all of them were ruled by the Welfen dynasty and still formed a common state. The centres of power moved in the meantime from Brunswick and Lüneburg to Celle and Wolfenbüttel.
One of the collateral lines was the line of the dukes of Calenberg, who managed to gain all the territory of the former duchy except for the Wolfenbüttel line. The city of Hanover was the residence of the Calenberg line. Calenberg (sometimes also called Calenberg-Celle) was made an electorate by the Holy Roman Emperor in 1692. It was then known under many different names (Brunswick-Lüneburg, Calenberg, Hanover), but eventually became the state of Hanover.
The Wolfenbüttel line retained its independence from Hanover. The Congress of Vienna of 1815 turned it into an independent country under the name Duchy of Brunswick, with Wolfenbüttel as its capital. The Duchy of Brunswick joined the German Empire in 1871 and became a part of the Weimar Republic in 1919. In 1946, it was made a part of the newly founded state of Lower Saxony. Brunswick had an area of 3690 km² and 580,000 inhabitants (1939).