The Kingdom of Romania was neutral for the first two years of World War I, entering on the side of the Allied powers from 27 August 1916 until Central Power occupation led to the Treaty of Bucharest in May 1918, before reentering the war on 10 November 1918. It had the most significant oil fields in Europe, and Germany eagerly bought its petroleum, as well as food exports.

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.