United Principalities The United Principalities of Moldavia and Wallachia , also known as The Romanian United Principalities , was the official name of Romania following the 1859 election of Alexandru Ioan Cuza as the Ruling Prince or " domnitor " of both territories . On 24 January ( O . S . ) 1862 , the Principality of Moldavia and the Principality of Wallachia formally united to create the United Principalities . Alongside Transylvania , the principalities became the core of the Romania n nation state . A new constitution came into effect in 1866 giving the country the official name Romania , and on 14 March ( O . S . ) 1881 , it became the Kingdom of Romania . Background . As a historical term designating the pre - Union Principalities of Moldavia and Wallachia , sometimes including the Principality of Transylvania , the term " Romanian Principalities " dates back to the beginnings of modern Romanian history in the mid 19th century . It was subsequently used by Romanian historians as an alternative to the much older term " Romanian Lands " . English use of " Romanian Principalities " is documented from the second half of the 19th century . In the period between the late 18th century and the 1860s , Danubian Principalities was used , a term that sometimes included Serbia , but not Transylvania . In contrast , use of " Romanian Principalities " sometimes included Transylvania but never Serbia . History . The aftermath of the Russian Empire ' s defeat in the Crimean War brought the 1856 Treaty of Paris , which started a period of common tutelage for the Ottomans and a Congress of Great Power s – ( the United Kingdom of Great Britain and Ireland , the Second French Empire , the Kingdom of Piedmont - Sardinia , the Austrian Empire , Prussia , and , though never again fully , Russia . While the Moldavia - Wallachia unionist campaign , which had come to dominate political demands , was accepted with sympathy by the French , Russians , Prussians , and Sardinians , it was rejected by the Austrian Empire , and looked upon with suspicion by Great Britain and the Ottomans . Negotiations amounted to an agreement on a minimal formal union ; however , elections for the ad - hoc divan s in 1859 profited from an ambiguity in the text of the final agreement , which , while specifying two thrones , did not prevent the same person from occupying both thrones simultaneously and ultimately ushered in the ruling of Alexandru Ioan Cuza as " Domnitor " ( Ruling Prince ) over the United Romanian Principalities from 1862 onwards . Though internationally formally recognized only after the period of Cuza ' s reign , the Union was cemented by Ioan Cuza ' s unsanctioned interventions in the text of previous " Organic Law " . In addition , the circumstances of his deposition in 1866 , together with the rapid election of Prussian Prince Carol of Hohenzollern - Sigmaringen ( who was backed by the increasingly important Prussia ) and the Austro - Prussian War in the same time , made applying measures against the Union actually impossible . Following the 1877 - 8 war of independence , Romania shook off formal Ottoman rule , but eventually clashed with its Russian ally over its demand for the Budjak ( southern Bessarabia ) region . Ultimately , Romania was awarded Northern Dobruja in exchange for southern Bessarabia . The Kingdom of Romania subsequently emerged in 1881 with Prince Carol being crowned as King Carol I of Romania . The reign of Alexandru Ioan Cuza . Alexandru Ioan Cuza took steps to unify the administrations of the two Romanian Principalities and gain international recognition for the Union . He also adopted several reforms , including the secularization of church lands , introduction of free primary education , a French - inspired civil code and penal code as well as a limited agrarian reform and one in the army . Opposition from the large - land - owners dominated parliament to Cuza resulted in a coup against him in 1864 . He subsequently instituted authoritarian rule but his popular support , strong at the time of the coup , gradually waned as the land reform failed to bring prosperity to the peasant majority . Cuza was forced to abdicate in 1866 by the two main political groups , the Conservatives and the Liberals , who represented the interests of former large - land - owners . Although the event sparked some anti - unionist turmoil in Cuza ' s native province of Moldavia , it was quickly suppressed by the central authorities . The reign of Carol I as Prince . The new governing coalition appointed Carol of Hohenzollern - Sigmaringen as the new Ruling Prince of Romania in a move initially rejected by the European powers but later on accepted . In the first year of Carol ' s reign Romania adopted its first constitution . This instrument provided for a hereditary constitutional monarchy , with a Parliament being elected through censitary suffrage although the country remained under Ottoman suzerainty . Carol was not unanimously accepted , and a rise in republican sentiment culminated with an uprising in Ploieşti in 1870 and a revolt in Bucharest in 1871 , both of which were quelled by the army . In April 1877 , in the wake of a new Russo - Turkish war , Romania signed a convention by which Russian troops were allowed to pass through Romanian territory in their advance towards the Ottoman Empire . On May 9 , the Romanian parliament declared the independence of the principality , and joined the war on the Russian side . After several Romanian victories south of the Danube and the ultimate victory of the Russian - led side in the war , the European powers recognized Romania ' s independence under the 1878 Treaty of Berlin . Nevertheless , Romania was made to exchange Southern Bessarabia for Northern Dobruja , and allow non - Christians living in Romania access to Romanian citizenship . In 1881 , the country ' s parliament proclaimed Romania a kingdom .