Ernst August (17 November 1887-30 January 1953), reigning Duke of Brunswick-Lüneburg (2 November 1913-8 November 1918), was a grandson of King George V of Hanover, whom the Prussians deposed in 1866. His great-grandfather, Prince Ernest Augustus, Duke of Cumberland, the fifth son of King George III of the United Kingdom, became king of Hanover in 1837 because Salic Law barred Queen Victoria from reigning in Germany. The last reigning monarch of the House of Hanover, Ernst August was a direct descendant of Heinrich der Löwe.

His Royal Highness Prince Ernst August Christian Georg of Hanover and Cumberland, Prince of Great Britain and Ireland, was born at Penzing near Vienna, the sixth and youngest child of Ernst August of Hanover, 3rd Duke of Cumberland and his wife, the former Princess Thyra of Denmark.¹ His father, the exiled Crown Prince of Hanover, succeeded as the Duke of Cumberland in the peerage of Great Britain in 1878. The younger Prince Ernst August became heir apparent to the dukedom of Cumberland upon the deaths of his two elder brothers (George and Christian). He received a commission in a Bavarian cavalry regiment.

In 1884, the reigning Duke of Brunswick-Wolfenbüttel, a distant cousin, died and the Duke of Cumberland claimed to succeed to that territory. However, the Reich chancellor Otto von Bismarck, managed to get the Federal Council (Bundesrat) of the German Empire to exclude him from the succession. Bismarck did this because the duke had never formally renounced his claims to the kingdom of Hanover, which had been annexed to Prussia in 1866. Instead, Prince Albrecht of Prussia became the regent of Brunswick. After Prince Albrecht's death in 1907, the duke was again excluded from the Brunswick succession and the regency continued.

On 24 May 1913, Prince Ernst August of Cumberland married Princess Viktoria Luise of Prussia (13 September 1892-11 December 1980), the only daughter of the German emperor Wilhelm II. The marriage ended the decades-long rift between the House of Hohenzollern and the House of Hanover, that began with the Austro-Prussian War in 1866 (in which Hanover sided with the losing Austria). The wedding of Prince Ernst August and Princess Viktoria Luise was also the last great gathering of European sovereigns (many descended from Queen Victoria and King Christian IX of Denmark) before the outbreak of the First World War. In addition to the German Emperor and Empress and the Duke and Duchess of Cumberland, King George V and Queen Mary of Great Britain, Tsar Nicholas II and Empress Alexandra Feodrovna of Russia attended. Upon the announcement of his betrothal of Princess Viktoria Luise in February 1913, Prince Ernst August took an oath of loyalty to the German emperor and accepted a commission as a cavalry captain and company commander in the Zieten Hussars, a Prussian Army regiment in which his grandfather (George V) and great grandfather (Ernst August I) had been colonels.

On 27 October 1913, the Duke of Cumberland formally renounced his claims to the (former) kingdom of Hanover and the duchy of Brunswick in favor of his surviving son. The following day, the Federal Council voted to allow Prince and Princess Ernst August of Cumberland to become the reigning Duke and Duchess of Brunswick-Lüneburg. The new Duke of Brunswick, who received a promotion to colonel in the Zieten Hussars, formally took possession of his duchy on 1 November. During World War I, he rose to the rank of major-general. On 8 November 1918, he was forced to abdicate his throne along with the other German kings, grand dukes, dukes, and princes. He retired to live on his various estates.

The Duke and Duchess of Brunswick-Lüneburg had five children ²:

• Prince Ernst August IV of Hanover (18 March 1914-9 December 1987); married (1) 1951, Princess Ortrud of Schleswig-Holstein-Sonderburg-Glücksburg (19 December 1925-6 February 1980), and had issue; and (2), 1980 Monika, Baroness zu Solms-Laubach (8 August 1929)-).
• Prince Georg of Hanover (25 March 1915-); married 1946, Princess Sophie of Greece and Denmark (26 June 1914-24 November 2001), and had issue.
• Princess Frederika of Hanover (18 April 1917-6 February 1981); married 1938 King Paul I of the Hellenes (14 December 1901-6 March 1964), and had issue.
• Prince Christian of Hanover (1 September 1919-10 December 1981); married 1963 (divorced 1976) Mireille Dutry (10 January 1946-), and had issue.
• Prince Welf-Heinrich of Hanover (11 March 1923-); married 1960 Princess Alexandra zu Ysenburg und Büdingen (23 October 1937-), and has issue.

The former Duke of Brunswick-Lüneburg died at Marienburg Castle in 1953.

Footnotes

¹ Under settled practice dating to 1714, as a male-line great grandchild of King George III, Prince Ernst August III of Hanover also held the title of Prince of Great Britain and Ireland with the style of Highness. In the Court Circular printed in The Times and in the London Gazzette, he was frequently styled Prince Ernest Augustus of Cumberland.

² By Royal Warrant of 17 June 1914, King George V granted the eldest son and any children thereafter born to Prince Ernst August of Hanover, then reigning Duke of Brunswick-Lüneburg, the title of Prince (or Princess) of Great Britain and Ireland with the style Highness. The provisions of this Royal Warrant ceased with George V's Letters Patent of 30 November 1917, and Hanoverian princes and princesses born after this date were no longer allowed the title Prince of Great Britain and Ireland with the style Highness. However, in 1931, the former Duke of Brunswick-Lüneburg, as head of the House of Hanover and the senior male-line descendant of George III, issued a decree stating that the members of the former Hanoverian royal family would continue to bear the title of Prince (or Princess) of Great Britain and Ireland with the style of Royal Highness. This decree had no legal effect in the United Kingdom, although no British sovereigns since have attempted to stop this practice on the part of the former Hanoverian royal family. The members of the House of Hanover continue to seek the British sovereign's approval when they marry, in accordance with the Royal Marriages Act 1772. In 1999, prior to the wedding of Ernst August of Hanover (b. 1954) to Princess Caroline of Monaco, the couple received an official blessing from the reigning British monarch, Queen Elizabeth II.de:Ernst August (Braunschweig-Lüneburg) nl:Ernst August van Brunswijk