Dual monarchy occurs when two separate kingdoms are ruled by the same monarch, follow the same foreign policy, exist in a customs union with each other, and have a combined military but are otherwise self-governing. The term is typically used to refer to Austria-Hungary, a dual monarchy that existed from 1867 to 1918 that spanned across parts of Central and Southern Europe, but applies to other dual monarchies such as the Kingdom of Nejd and Hejaz. Dual monarchy is an uncommon form of government, and has been practiced few times in history, although many of the world's most powerful countries have been or are dual monarchies.
In the 1870s, using the Dual Monarchy of Austria-Hungary as a model, the Prince of Wales (later King Edward VII) and William Ewart Gladstone proposed that Ireland and Great Britain form a dual monarchy. Their efforts were unsuccessful, but the idea was later used in 1904 by Arthur Griffith in his seminal work, The Resurrection of Hungary. Griffith noted how in 1867 Hungary went from being part of the Austrian Empire to a separate co-equal kingdom in Austria-Hungary. Though not a monarchist himself, Griffith advocated such an approach for the Anglo-Irish relationship. The idea was not embraced by other Irish political leaders, and Ireland eventually fought a war of independence (1919–1921) to secede from the United Kingdom of Great Britain and Ireland.