The territory of Hanover had earlier been a principality within the
Holy Roman Empire, before being elevated into an Electorate in 1708.
(See prince-elector.) Hanover was formed by the union of several
dynastic divisions of the Duchy of Brunswick-Lüneburg, with the
sole exception of Brunswick-Wolfenbüttel.

Between 1714 and 1837 it was joined in a personal union, first with
the Kingdom of Great Britain and the Kingdom of Ireland, and then
from 1801 with the United Kingdom of Great Britain and Ireland. In
1803 the electorate was occupied by French and Prussian troops, and
following the Peace of Tilsit in 1807, its territories together with
territories ceded from Prussia was created into the Kingdom of
Westphalia ruled by Napoleon's brother Jérôme Bonaparte. French
control lasted until October 1813 when the territory was overrun by
Russian cossack troops, and the Battle of the Nations at Leipzig
later the same month spelled the definitive end to the Napoleonic
client state as well as the entire Confederation of the Rhine, after
which the House of Hanover was restored as rulers.

The prince-electors, formerly vassals within the Holy Roman Empire,
were elevated to monarchs of an independent kingdom at the Congress
of Vienna in 1814. This was conducted under the supervision and
advice of Baron Münster, head of the German Chancery in London.

During the British Regency and the reigns of the kings George IV
and later William, their younger brother Prince Adolphus, Duke of
Cambridge officiated as Viceroy of Hanover(1816-1837), representating
the King usually living in England. When Adolphus' niece, Queen
Victoria, daughter of his late elder brother Prince Edward, Duke of
Kent and Strathearn, succeeded to the British throne on 20 June 1837,
the 123-year personal union of the crowns of Great Britain (or the
United Kingdom of Great Britain and Ireland from 1801, respectively)
and Hanover (or the Electorate of Brunswick-Lüneburg before 1814,
respectively) ended. Adolphus' other elder brother became King as
Ernest Augustus I of Hanover and Adolphus 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 voting in favour 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 Prussia, where
it became the Province of 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 up until it was
banned by the National Socialist government. After George V fled Hanover,
he raised forces loyal to him in the Netherlands called the Guelphic
Legion. They were eventually disbanded in 1870.