After the 1907 peasant uprising, foreign events shaped Romania's political agenda. In 1908 Austria annexed Bosnia, a clear indication that Vienna sought to destroy Serbia. A year later Ionel Bratianu, son of the former Liberal Party leader, became Romania's prime minister. Bratianu feared that Bulgarian expansion might upset the Balkan balance of power and sought compensation for any potential Bulgarian gains at the Ottomans' expense.

Then in October 1912, the First Balkan War erupted. Serbia, Montenegro, and Greece scored quick victories over Ottoman forces, and Bulgarian forces drove to within thirty-three kilometers of Constantinople. Romania called on Sofia to hand over the fortress of Silistra; Bulgaria's foreign minister, however, offered only minor border changes, which excluded Silistra, and assurances for the rights of the Kutzovlachs in Macedonia and northern Greece. After the war, Romania threatened to occupy Bulgarian territory, but a British proposal for arbitration prevented hostilities. The resulting May 1913 Protocol of St. Petersburg awarded Romania control of Silistra; the protocol did not satisfy Bucharest's appetite for territory, however, and Sofia considered the award excessive.

On June 28, 1913, the Second Balkan War broke out when Bulgaria launched an unsuccessful surprise attack on Serbia and Greece. The Ottomans joined in the fighting against Bulgaria, and Romania's army marched into southern Dobruja before turning toward Sofia. The warring states signed an armistice on July 30, 1913, and in the subsequent Treaty of Bucharest, Romania retained Silistra and other strategic areas of Dobruja. During the invasion of Bulgaria, large numbers of Romanian soldiers saw firsthand Bulgaria's abundant peasant holdings and more advanced farming methods and noted the absence of wealthy landowners and rapacious middlemen. Bratianu's Liberal Party tapped the resulting impatience of Romania's peasantry by making land and franchise reform the thrust of its new program; they proved an unstoppable combination against the Conservatives. In January 1914, the Liberals rose to power and convoked a constituent assembly to elaborate agrarian and electoral reform programs.

When Bratianu became premier, he learned that Charles had renewed the secret treaty with the other Central Powers in 1913 despite the fact that the king knew the treaty would enjoy no popular support because of Hungary's continuing efforts to Magyarize Transylvania's Romanians. On June 28, 1914, a Bosnian Serb assassinated Archduke Franz Ferdinand, the heir to the Austrian throne and the Dual Monarchy's most ardent supporter of the rights of Transylvania's Romanians. Within days Austria presented Serbia with an ultimatum that made war inevitable. At first, King Charles felt the secret treaty did not bind Romania to declare war on Serbia for a quarrel that Austria-Hungary had provoked with its ultimatum. The Central Powers, eager to have Charles mobilize Romania's forces against Russia, evoked the king's German ancestry and tempted him with a promise to restore Bessarabia; at the same time, Russia offered Transylvania to Romania if it would join the Triple Entente, the military alliance of Great Britain, France, and Russia set up to counter the Central Powers. At a meeting of government and opposition-party leaders deciding Romania's course of action, Charles advocated joining the Central Powers. But upon hearing about Charles' secret, unconstitutional treaty, virtually all the government leaders rejected the king's proposal and opted for a wait-and-see policy. Romanian public opinion adamantly backed the French, and Bucharest crowds cheered after the French checked the German advance at the Marne River.

King Charles, infirm and disconsolate that Romania did not honor his secret treaty, died in October 1914. If it had not been for the war, Romanians would have grieved for the end of a fortyeight -year reign that had brought them the most prosperous and peaceful period in their entire history. Charles's successor, Ferdinand (1914-27), and Bratianu chose to conserve Romania's resources and continue playing a waiting game until they could discern the outcome of the war. In November Hungary tried to dissipate Romania's animosity by announcing a number of reforms benefiting Transylvania's ethnic Romanians, but even Germany termed the measures inadequate. In October 1915, Romania's rival, Bulgaria, joined the Central Powers and, in unison with Germany, attacked Serbia. Russian victories in Galicia in 1916, Allied promises of territory, and fear of Germany finally convinced Romania to join the war on the side of Britain, Russia, France, and Italy. On August 27, 1916, Romania declared war on Austria-Hungary. Confident of victory, Romanian troops crossed into Transylvania. Their campaign stalled, however, and German and Austrian forces counterattacked, drove the Romanian army and thousands of refugees back over the Carpathian passes, and in December occupied Bucharest. Bulgarian forces also invaded from across the Danube, and Russian reinforcements sent to Romania's aid proved feckless. Meanwhile, Ferdinand and his ministers fled to Iasi, where the Romanian army regrouped under a French military mission, achieved several victories over Central Power forces, and held a line along the Siret River.

In February 1917, revolution erupted in Russia's capital, Petrograd. In an effort to preempt the appeal of Bolshevik propaganda, the Romanian government in July 1917 enacted a land reform program and an election law providing for universal suffrage, proportional representation, and obligatory participation in elections. By late summer, Russia's defenses had collapsed, and its soldiers were openly fraternizing with the enemy. In November the Bolsheviks staged a coup d'état that overthrew Russia's provisional government. Romania's leaders refused to participate in the subsequent German-Soviet armistice negotiations; once the Treaty of Brest-Litovsk was signed, however, Romania had little choice but to agree to a preliminary armistice. In December Romanian nationalists in Bessarabia convened a representative national assembly that proclaimed the creation of the Democratic Federative Moldavian Republic and appealed to the Iasi government and Entente countries for help in repulsing Bolshevik forces. In April 1918, the Bessarabian assembly requested annexation to Romania, and Romanian troops entered the province.

A new Romanian premier, the pro-German Alexandru Marghiloman, signed the Treaty of Bucharest with the Central Powers on May 7, 1918. Under the treaty, Romania lost all of Dobruja to Bulgaria and a joint administration of the Central Powers; Hungary gained territory in the Carpathians; Romania had to compensate the Central Powers for debts and damages; and the Central Powers claimed a nine-year monopoly on Romania's agricultural output and assumed control of the Danube and Romania's oilfields, railroads, wharves, and other economic assets. The Central Powers intended to ruin Romania's economy, and Hungary launched an all-out effort to create a wholly Magyarized zone along Transylvania's Romanian border and undermine the Orthodox and Uniate churches.

By mid-1918 the tide of the war had turned and engulfed the Central Powers. Bulgaria soon capitulated, Austria-Hungary was disintegrating, and Germany was retreating on the Western Front. The leaders of Transylvania's National Party met and drafted a resolution invoking the right of self-determination, and a movement began for the unification of Transylvania with Romania. In November near-anarchy gripped Hungary, and the Romanian National Central Council, which represented all the Romanians of Transylvania, notified the Budapest government that it had assumed control of twenty-three Transylvanian counties and parts of three others. A similar Romanian national council in northern Bukovina announced its union with Romania, and Bessarabia's government also voted for unification. In Romania itself, King Ferdinand appointed a new government that repealed all laws enacted under Marghiloman's administration. On November 8, Romania declared war on Germany and forced enemy troops from Walachia. The king returned to Bucharest on November 30, and Romanian units occupied most of Transylvania by December 1. A mass assembly later that month in Alba Iulia (southern Transylvania), passed a resolution calling for unification of all Romanians in a single state.

Data as of July 1989