The Kingdom of Romania (Romanian: Regatul României) was a constitutional monarchy that existed from 25 March [O.S. 13 March] 1881 with the crowning of prince Karl of Hohenzollern-Sigmaringen as King Carol I (thus beginning the Romanian royal family), until 30 December 1947 with the abdication of King Michael I and the Romanian Parliament proclamation of the Romanian People's Republic.
From 1859 to 1877, Romania evolved from a personal union of two principalities: (Moldavia and Wallachia) called the Unification of Moldavia and Wallachia also known as "The Little Union" under a single prince to an autonomous principality with a Hohenzollern monarchy. The country gained its independence from the Ottoman Empire during the 1877–1878 Russo-Turkish War (known locally as the Romanian War of Independence), after which it was forced to cede the southern part of Bessarabia in exchange for Northern Dobruja. The kingdom's territory during the reign of King Carol I, between 13 (O.S.) / 25 March 1881 and 27 September (O.S.) / 10 October 1914 is sometimes referred to as the Romanian Old Kingdom, to distinguish it from "Greater Romania", which included the provinces that became part of the state after World War I (Bessarabia, Banat, Bukovina, and Transylvania).
With the exception of the southern halves of Bukovina and Transylvania, these territories were ceded to neighboring countries in 1940, under the pressure of Nazi Germany or the Soviet Union. Following the abolishment of the 1923 constitution by King Carol II in 1938, the Kingdom of Romania became an absolute monarchy, only to become a military dictatorship under Ion Antonescu in 1940 after the forced abdication of King Carol II, with his successor, King Michael I being a figurehead with no effective political power. The country's name was changed to Legionary Romania.
The disastrous World War II campaign on the side of the Axis powers led to King Michael's Coup against Ion Antonescu in 1944, as a result of which the Kingdom of Romania became a constitutional monarchy again and switched sides to the Allies, recovering Northern Transylvania. The influence of the neighbouring Soviet Union and the policies followed by Communist-dominated coalition governments ultimately led to the abolition of the monarchy, with Romania becoming a Soviet satellite state as the People's Republic of Romania on the last day of 1947.
The 1859 ascendancy of Alexandru Ioan Cuza as prince of both Moldavia and Wallachia under the nominal suzerainty of the Ottoman Empire united an identifiably Romanian nation under a single ruler. On 24 January (O.S.) / 5 February 1862, the two principalities were formally united to form the Principality of Romania, with Bucharest as its capital.
On 11 (O.S.) / 23 February 1866 a so-called "monstrous coalition", composed of Conservatives and radical Liberals, forced Cuza to abdicate. The German prince (and Prussian officer) Karl Eitel Friedrich Zephyrinus Ludwig von Hohenzollern-Sigmaringen was appointed as Prince of Romania, in a move to assure Prussian backing to unity and future independence. He immediately adopted the Romanian spelling of his name, Carol, and his cognatic descendants would rule Romania until the overthrow of the monarchy in 1947.
For more than a decade after the formal union of the two principalities, Romania was still nominally a vassal of the Ottoman Empire. However, this was increasingly a legal fiction. Romania had its own flag and anthem, and from 1867 had its own currency as well. Following the Russo-Turkish War of 1877–1878, Romania was recognized as an independent state by the Treaty of Berlin, 1878 and acquired Dobruja, although it was forced to surrender southern Bessarabia (Budjak) to Russia. On 15 March 1881, as an assertion of full sovereignty, the Romanian parliament raised the country to the status of a kingdom, and Carol was crowned king on 10 May.
The new state, squeezed between the Ottoman, Austro-Hungarian, and Russian Empires, with Slavic populations on its southwestern, southern, and northeastern borders, the Black Sea due east, and Hungarian neighbours on its western and northwestern borders, looked to the West, particularly France, for its cultural, educational, and administrative models.
Abstaining from the Initial Balkan War against the Ottoman Empire, the Kingdom of Romania entered the Second Balkan War in June 1913 against the Tsardom of Bulgaria. 330,000 Romanian troops moved across the Danube and into Bulgaria. One army occupied Southern Dobruja and another moved into northern Bulgaria to threaten Sofia, helping to bring an end to the war. Romania thus acquired the ethnically mixed territory of Southern Dobruja, which it had desired for years.
In 1916 Romania entered World War I on the Entente side. Romania engaged in a conflict against Bulgaria but as a result Bulgarian forces, after a series of successful battles, regained Dobruja, which had been previously ceded from Bulgaria by the treaty of Bucharest and the Berlin congress. Although the Romanian forces did not fare well militarily, by the end of the war the Austrian and Russian empires were gone; various assemblies proclaimed as representative bodies in Transylvania, Bessarabia and Bukovina decided on union with Romania. In 1919 by the Treaty of Saint-Germain and in 1920 by the Treaty of Trianon most of the territories claimed were assigned to Romania.
The Romanian Old Kingdom (Romanian: Vechiul Regat or just Regat; German: Regat or Altreich) is a colloquial term referring to the territory covered by the first independent Romanian nation state, which was composed of the Danubian Principalities – Wallachia and Moldavia. It was achieved when, under the auspices of the Treaty of Paris (1856), the ad hoc Divans of both countries – which were under Imperial Ottoman suzerainty at the time – voted for Alexander Ioan Cuza as their prince, thus achieving a de facto unification. The region itself is defined by the result of that political act, followed by the inclusion of Northern Dobruja in 1878, the proclamation of the Kingdom of Romania in 1881, and the annexation of Southern Dobruja in 1913.
The term came into use after World War I, when the Old Kingdom was opposed to Greater Romania, which included Transylvania, Banat, Bessarabia, and Bukovina. Nowadays, the term is mainly of historical relevance, and is otherwise used as a common term for all regions in Romania included in both the Old Kingdom and present-day borders (namely: Wallachia, Moldavia, and Northern Dobruja).
Romania delayed in entering World War I, but ultimately declared war on the Central Powers in 1916. The Romanian military campaign ended in stalemate when the Central Powers quickly crushed the country's offensive into Transylvania and occupied Wallachia and Dobruja, including Bucharest and the strategically important oil fields, by the end of 1916. In 1917, despite fierce Romanian resistance, especially at the Battle of Mărășești, due to Russia's withdrawal from the war following the October Revolution, Romania, being almost completely surrounded by the Central Powers, was forced to also drop from the war, signing the Armistice of Focșani and next year, in May 1918, the Treaty of Bucharest. But after the successful offensive on the Thessaloniki front which put Bulgaria out of the war, Romania's government quickly reasserted control and put an army back into the field on 10 November 1918, a day before the war ended in Western Europe. Following the proclamation of the union of Transylvania with the Kingdom of Romania on 1 December 1918 by the representatives of Transylvanian Romanians gathered at Alba Iulia, Transylvania was soon united with the Kingdom, as was Bessarabia earlier in 1918, since the power vacuum in Russia caused by the civil war there allowed the Sfatul Țării, or National Council, to proclaim the union of Bessarabia with Romania. War with the Hungarian Soviet Republic in 1919 resulted in the occupation of Budapest by Romanian troops and the end of Béla Kun's Bolshevik regime.