Romania in World War I

The Kingdom of Romania remained neutral throughout the first two years of World War I. They eventually entered the conflict on the side of the Entente from 27 August 1916 until insurmountable pressure from Central Powers - which had occupied two thirds of the country - led to an armistice being signed on 9 December 1917. Six months later, a crippling peace treaty was imposed on Romania, which the government ratified. King Ferdinand I refused to promulgate the treaty, hoping for an Allied victory on the Western Front. As the Central Power war efforts collapsed, Romania re-entered the war on 10 November 1918.

Romania was still a burgeoning state with great territorial ambitions at the onset of the war. It had achieved its independence following the Russo-Turkish War of 1877–1878, although millions of ethnic Romanians continued to reside outside the new nation's borders, particularly in Transylvania and Bessarabia, which were part of Austria-Hungary and Russia respectively. The Romanian monarchy, which was formed by members of the Hohenzollern dynasty, who were of Germanic origin, was sympathetic towards the cause of the Central Powers. The nation's political elite and the majority of the public favoured the Entente, as joining them would allow Romania to take Transylvania from Austria-Hungary, a region rich with natural resources and inhabited by a Romanian majority. Because of this social division and the general feeling that Romania still wasn't fully prepared for a war against a great power, the Romanians initially opted for neutrality.

At the outbreak of hostilities, the Austro-Hungarian Empire invoked a casus foederis on Romania and Italy linked to the secret treaty of alliance since 1883. However, both Italy and Romania refused to honor the treaty on the grounds that it was not a case of casus foederis because the attacks on Austria were not "unprovoked", as stipulated in the treaty of alliance. In August 1916, Romania received an ultimatum to decide whether to join the Entente "now or never". Under the pressure of the ultimatum, the Romanian government agreed to enter the war on the side of the Entente, although the situation on the battle fronts was not favorable.

The Romanian campaign was part of the Eastern Front of World War I, with Romania and Russia allied with Britain and France against the Central Powers of Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria. Fighting took place from August 1916 to December 1917 across most of present-day Romania, including Transylvania, which was part of the Austro-Hungarian Empire at the time, as well as in Southern Dobruja, which is currently part of Bulgaria.

The Romanian campaign plan (Hypothesis Z) consisted in attacking Austria-Hungary in Transylvania, while defending Southern Dobruja and Giurgiu from Bulgaria in the south. Despite initial successes in Transylvania, after German divisions started aiding Austria-Hungary and Bulgaria, the Romanian forces (aided by Russia) suffered massive setbacks, and by the end of 1916 out of the territory of the Romanian Old Kingdom only Western Moldavia remained under the control of the Romanian and Russian armies.

After several defensive victories in 1917 at Mărăști, Mărășești, and Oituz, with Russia's withdrawal from the war following the October Revolution, Romania, almost completely surrounded by the Central Powers, was also forced to drop out of the war. It signed the Treaty of Bucharest with the Central Powers in May 1918. Under the terms of the treaty, Romania would lose all of Dobruja to Bulgaria, all the Carpathian passes to Austria-Hungary and would lease all of its oil reserves to Germany for 99 years. However, the Central Powers recognized Romania's union with Bessarabia who had recently declared independence from the Russian Empire following the October Revolution and voted for union with Romania in April 1918. The parliament signed the treaty, but King Ferdinand refused to sign it, hoping for an Allied victory on the western front. In October 1918, Romania renounced the Treaty of Bucharest and on 10 November 1918, one day before the German armistice, Romania re-entered the war after the successful Allied advances on the Macedonian front and advanced in Transylvania. The next day, the Treaty of Bucharest was nullified by the terms of the Armistice of Compiègne.