Russia withdrew from Walachia and Moldavia in 1851 but returned yet again in the summer of 1853, thus precipitating the Crimean War. In 1854 Franz Joseph and the sultan forced Tsar Nicholas I to withdraw his troops from the principalities, and imperial and Ottoman soldiers soon occupied them. Russia's defeat in the Crimea forced the tsar to seek peace, affirmed in 1856 by the Treaty of Paris. De jure Ottoman suzerainty over the principalities continued after the treaty, which abolished the Russian protectorate and replaced it with a joint European guarantee. The treaty also freed navigation on the Danube and forced Russia to cede part of southern Bessarabia, which included control of the river's mouth, to Moldavia.

The year 1856 began the active campaign for union of Walachia and Moldavia. The movement had the support of France, because many Romanian revolutionaries took refuge there after 1848 and lobbied Napoleon III to press for unification; Austria, Britain, and the Ottomans, however, opposed the unification effort, while Russia opted to let the Romanians decide. In 1857 the Porte manipulated an election of delegates to special assemblies charged with discussing unification; the few voters casting ballots elected representatives opposing union. An international crisis followed, and Napoleon III, with Russian and British support, finally pressured the Ottomans to nullify the results and hold new, untainted elections, which returned a huge majority of delegates in favor of unification. These delegates immediately called for autonomy, a constitutional government, and a foreign prince to rule the unified principalities. Despite the election results, an international conference in Paris in 1858 reaffirmed separation of Walachia and Moldavia under Ottoman sovereignty, but it allowed for a common coinage and uniform laws and titled the two states the "United Principalities." The Romanians themselves overcame the imposed separation in 1859 when the separate assemblies at Bucharest and Iasi unanimously elected the same man, Alexandru Ioan Cuza, governor of both principalities. Distracted by war in Italy, the leading European nations yielded to a fait accompli and accepted unification, and Cuza (1859-66) became prince.

Data as of July 1989