The territory of Hanover had earlier been a principality within the Holy Roman Empire before being elevated into an electorate in 1708, when Hanover was formed by union of the dynastic divisions of the Duchy of Brunswick-Lüneburg, excepting the Principality of Brunswick-Wolfenbüttel.
After his accession in 1714, George Louis of the House of Hanover ascended the throne of Great Britain as George I, and Hanover was joined in a personal union with Great Britain. Descendants of Hanoverians who fought alongside the British in the War of 1812 remain in Canada. In 1803, Hanover was conquered by the French and Prussian armies in the Napoleonic Wars. The Treaties of Tilsit in 1807 joined it to territories from Prussia and created the Kingdom of Westphalia, ruled by Napoleon's youngest brother Jérôme Bonaparte. French control lasted until October 1813 when the territory was overrun by Russian Cossacks. The Battle of Leipzig shortly thereafter spelled the definitive end of the Napoleonic client states, and the electorate was restored to the House of Hanover.
The terms of the Congress of Vienna in 1814 not only restored Hanover, but elevated it to an independent kingdom with its Prince-Elector, George III of Great Britain, as King of Hanover. The new kingdom was also greatly expanded, becoming the fourth-largest state in the German Confederation (behind Prussia, Austria and Bavaria) and the second-largest in north Germany.
Under George III's six-year reign, he never visited the Kingdom, having succumbed to dementia prior to the elevation of Hanover, it is unlikely he ever understood that he had gained an additional kingship nor did he take any role in its governance. Functional administration of Hanover was usually handled by a viceroy, which during the later years of George III's reign and the reigns of kings George IV and William IV from 1816 to 1837, was Adolph Frederick, George III's youngest surviving son. When Queen Victoria succeeded to the British throne in 1837, the 123-year personal union of Great Britain and Hanover ended. Salic law operated in Hanover, excluding accession to the throne by a female while any male of the dynasty survived; thus instead of Victoria, her uncle in the male-line of the House of Hanover, Ernest Augustus, now the eldest surviving son of George III, succeeded to the throne of the new kingdom as King of Hanover; Adolph Frederick the younger brother, and long-time Viceroy, returned to Britain.
During the Austro-Prussian War (1866), Hanover attempted to maintain a neutral position, along with some other member states of the German Confederation. Hanover's vote in favor of the mobilisation of Confederation troops against Prussia on 14 June 1866 prompted Prussia to declare war. The outcome of the war led to the dissolution of Hanover as an independent kingdom and it was annexed by the Kingdom of Prussia, becoming the Prussian Province of Hanover. Along with the rest of Prussia, it became part of the German Empire in 1871.
After George V fled Hanover in 1866, he raised forces loyal to him in the Netherlands, called the Guelphic Legion. They were eventually disbanded in 1870. Nevertheless, George refused to accept the Prussian takeover of his realm and claimed he was still the legitimate king of Hanover. His only son, Ernest Augustus, Crown Prince of Hanover, inherited this claim upon George's death in 1878. Ernest Augustus was also first in line to the throne of the Duchy of Brunswick, whose rulers had been a junior branch of the House of Hanover. In 1884, that branch became extinct with the death of William, a distant cousin of Ernest Augustus. However, since Ernest Augustus refused to renounce his claim to annexed Hanover, the Bundesrat of the German Empire ruled that he would disturb the peace of the empire if he ascended the throne of Brunswick. As a result, Brunswick was ruled by a regency until 1913, when his son, also named Ernest Augustus, married the German Emperor's daughter, Princess Viktoria Luise and swore allegiance to the German Empire. The Duke then renounced his claim to Brunswick in favor of his son, and the Bundesrat allowed the younger Ernest Augustus to take possession of Brunswick as a kind of dowry compensation for Hanover.
The German-Hanoverian Party, which at times supported secession from the Reich, demanded a separate status for the province in the Reichstag. The party existed until banned by the Nazi government.
Revival and modern history
Map of Kingdom of Hanover (until 1866), recreated in 1946 as the State of Hanover.
With Prussia in agony and on the verge of official dissolution (1947), in 1946 Hanoverian politicians took advantage of the opportunity and advocated that the Control Commission for Germany - British Element (CCG/BE) revive Hanoverian statehood, reconstituting the Prussian Province of Hanover as the State of Hanover. The state saw itself in the tradition of the kingdom. Its Prime Minister, Hinrich Wilhelm Kopf, played a central role when the state of Lower Saxony was founded just a few months later by merging Hanover with several smaller states, with the city of Hanover as its capital. The former territory of Hanover makes up 85 percent of Lower Saxony's territory, and the state continues to use the old Hanover coat of arms.