A secret prewar member of the Triple Alliance, it stiffed their former associates when war cam in August 1914. Its leaders hemmed and hawed for two years and finally came into the war on the side of the Allies. Romania was then soundly thrashed in the campaign of 1916, suffering over 200,000 military casualties with two-thirds of the country falling under a harsh occupation regime. Its allies, especially Russia, stuck with a new front to defend, fared badly as well. Later when revolution came to Russia, the Romanians submitted to the onerous terms of the Peace of Bucharest (7 May 1918). Included were territorial losses, especially Dobrogea and the Carpathian passes; longtime concessions of Romanian natural resources; and sequential demobilization of their army.
Yet the worm turned one last time for them. After their enemies opened armistice negotiations [Bulgaria (29 September), Austria (4 November) and Germany (7 November)], the Romanian government re-mobilized and declared war on Germany, just hours before the armistice of 11 November. And somehow, Romania came out of the war as one of the victors.
The final settlement of the Paris Peace Conference awarded to Romania a frontier approximately 50 kilometers to the east of this line. It also sanctioned Romania’s acquisition of Bukovina, Bessarabia, and half of the Banat. This settlement virtually doubled Romania’s prewar territory and population, with attendant economic gains. However, it also brought with it dissatisfied minorities and hostile neighbors. In the Second World War, Bukovina and Bessarabia were lost, but what has remained to become contemporary Romania is a reasonable fulfillment of the aims for which the nation entered the First World War.
Source: "Romania and the Great War" by Glenn Torrey at Russia's Great War & Revolution.