Romania was a brief participant in World War 1. Although the country is situated in the volatile Balkans, and next door to Serbia, where the war originated, Romania initially stayed out of the conflict. It would have been better off if it had stayed out all together.
Romania entered the war in August 1916 on the side of the Allies, with the goal of seizing the province of Transylvania from the Austro-Hungarian Empire. Transylvania had a large ethnically Romanian population but was part of the Austrian Empire, that hodgepodge of nationalities and simmering ethnic rivalries.
However, Romania was at best a minor power and its poorly equipped and badly led armies fared terribly against the Central Powers. It suffered severe defeats at the hands of the Germans and Austrians, and was forced back onto its own territory. Despite having numerically inferior forces, the better led Germans and Austrians out maneuvered and out fought the Romanians.
By 1917 most of the country had been occupied by the Central Powers, including Bulgaria, and what was left of the Romanian armed forces and government hung on to Moldovia, bolstered by Russian troops. There, the front stabilized and the Romanians, with French and British support, worked to retrain and re-arm a new army with the goal of taking back their country. However when Russia collapsed and left the war in the aftermath of the Russian revolution, Romania sued for peace and signed and signed a treaty with Austria and Germany in December 1917.
Romania remained out of the war until the day before the German armistice in 1918, when Romania committed a dishonourable breach of that peace and declared war on Germany again, in order to gain a place at the peace conference and exact terms from the Germans.
Following the war, Romania did gain its prize of Transylvania which was annexed to Romania. But as the region also had a large Hungarian population, which resented the annexation, this led to conflict with the new Soviet Republic of Hungary. A new war in 1919 resulted in the partial occupation of Hungary by Romania and exemplified the fact that the so called War to End All Wars was only the forerunner of even greater conflicts to come.