Marie-Adélaïde, Grand Duchess of Luxembourg (Luxembourgish: Marie Adelheid; full list of given names: Marie Adelheid Thérèse Hilda Wilhelmine; 14 June 1894 – 24 January 1924), reigned from 1912 to 1919. She was the first Grand Duchess regnant of Luxembourg (after five Grand Dukes), its first female monarch since Duchess Maria Theresa (1740–1780, who was also Austrian Empress) and the first Luxembourgish monarch to be born within the territory since Count John the Blind (born in 1296).

Named as heir presumptive by her father Grand Duke William IV in 1907 to prevent a succession crisis due to his lack of a son, Marie-Adélaïde became Grand Duchess in 1912. She ruled through the First World War, and her perceived support for the German occupation forces led to great unpopularity in Luxembourg as well as neighbouring France and Belgium. In 1919, on the advice of Parliament and after enormous pressure from the Luxembourgish people, she abdicated on 15 January 1919 in favour of her younger sister Charlotte who managed to save the monarchy and the dynasty in a national referendum (28 September 1919).

After abdicating, Marie-Adélaïde retired in a monastery in Italy, before leaving due to ill health. She died of influenza in Germany on 24 January 1924 at the age of 29.

Marie-Adélaïde was born on 14 June 1894 in Berg Castle as the eldest child of Grand Duke William IV and his wife, Marie Anne of Portugal.

Since her father had six daughters and no sons, he proclaimed Marie-Adélaïde as the heir presumptive on 10 July 1907, in order to solve any succession crisis due to the use of Salic law in the monarchy.

Due to that same Salic Law, the elder branch of the House of Nassau, called Nassau-Weilburg (present-day Luxembourg-Nassau) inherited in 1890 the throne of Luxembourg from the younger branch called Nassau-Orange, which was not only supplying the Grand Dukes of Luxembourg from 1815 to 1890, but was also in a personal union (and still is) the reigning dynasty of the Kingdom of the Netherlands.

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