Romania had been allied with the Central Powers since 1882. When the war began, however, it declared its neutrality, arguing that because Austria-Hungary had itself declared war on Serbia, Romania was under no obligation to join the war. When the Entente Powers promised Romania large territories of eastern Hungary (Transylvania and Banat), that had a large Romanian population, in exchange for Romania’s declaring war on the Central Powers, the Romanian government renounced its neutrality, and on 27 August 1916 the Romanian army launched an attack against Austria-Hungary. The Romanian offensive was initially successful, pushing back the Austro-Hungarian troops in Transylvania, but a counter attack by the forces of the Central Powers defeated the Romanian army and as a result of the Battle of Bucharest the Central Powers occupied Bucharest on 6 December 1916. Fighting in Moldova continued in 1917 until an armistice was signed between the Central Powers and Romania on 9 December 1917.

In January 1918, Russia, allied to Romania, had to withdraw its troops from the Romanian front and Romanian forces established control over Bessarabia. Although a treaty was signed by the Romanian and the Bolshevik Russian government following talks between 5-9 March 1918 on the withdrawal of Romanian forces from Bessarabia within two months, on 27 March 1918 Romania attached Bessarabia to its territory, formally based on a resolution passed by the local assembly of the territory on the unification with Romania.

Romania officially made peace with the Central Powers signing the Treaty of Bucharest on 7 May 1918. Under that treaty Romania was obliged to cease war with the Central Powers. Romania made small territorial concessions for Austria-Hungary, ceding control of some passes in the Carpathian mountains and granted oil concessions for Germany. On the other hand, the Central Powers recognized the sovereignty of Romania over Bessarabia. The treaty was renounced in October 1918 by the Alexandru Marghiloman government and Romania nominally re-entered the war on 10 November 1918. The next day, the Treaty of Bucharest was nullified by the terms of the Armistice of Compi├Ęgne. Total Romanian deaths from 1914 to 1918, military and civilian, within contemporary borders, were estimated at 748,000.