Born in Sigmaringen in southwestern Germany, Prince Ferdinand of Hohenzollern-Sigmaringen became heir to the throne of his childless uncle, King Carol I of Romania in November 1888, following the renunciations of his father and elder brother. In 1893, Crown Prince Ferdinand married Princess Marie of Edinburgh, who was a granddaughter of Queen Victoria of the United Kingdom and of Czar Alexander II of Russia. They had three sons (one of whom died in infancy) and three daughters. Ferdinand succeeded his uncle as King of Romania on 10 October 1914, reigning until his death on 20 July 1927.
Though a member of a cadet branch of Germany's ruling Hohenzollern imperial family, he presided over his country's entry into World War I on the side of the Entente powers against Germany and Austria-Hungary (27 August 1916), seeking thereby to add the three million ethnic Romanians of Hungarian-ruled Transylvania and Austrian Bukovina to his 7.5 million subjects.
Despite Romania's disastrous defeat during the closing months of 1916, he became ruler of a greatly enlarged Romanian state in 1918-1920 following the Entente's victory and subsequent large Hungarian and Russian territorial losses, and was crowned king of greater Romania in a spectacular ceremony on 15 October 1922 at the historic princely seat of Alba Iulia.
Domestic political life during his reign was dominated by the conservative Liberal party led by the brothers Ion and Vintila Bratianu. The acquisition of Transylvania ironically enlarged the electoral base of the opposition, whose principal parties united in January 1925-October 1926 to form the National Peasant Party.