On August 27, 1916, after Romania declares war on Austria-Hungary, formally entering World War I, Romanian troops cross the border of the Austro-Hungarian Empire into the much-contested province of Transylvania.

By the outbreak of war in Europe in 1914, Romania had long been at odds with Austria-Hungary over the issue of territory—specifically Transylvania, which was ethnically Romanian but then part of Hungary. Seeing Russia’s success against Austria on the battlefields of the Eastern Front during the summer of 1916, Romania hoped to make an advantageous entry into the war in order to realize long-held dreams of territorial expansion and national unity. On August 18, 1916, the Romanian government signed a secret treaty with the Allies; by its terms, in the event of an Allied victory Romania would acquire Transylvania, up to the River Theiss, the province of Bukovina to the River Pruth, and the entire Banat region, all territory under Austro-Hungarian control. On August 27, Romania fulfilled its treaty obligation by declaring war against Austria-Hungary.

As Romanian troops opened a new front of the war in Transylvania, British forces pressured Germany on the Somme River, and Austria faltered against Russia in the east, Kaiser Wilhelm II of Germany briefly panicked, telling close confidantes that “The war is lost.” He regained perspective quickly, however, and moved to strengthen Germany’s defensive position, replacing Erich von Falkenhayn with Paul von Hindenburg as chief of the German General Staff on August 28. Within two weeks, at a conference that included Turkish and Bulgarian leaders, Wilhelm sanctioned the creation of a Supreme War Command, effectively giving Hindenburg command of all the armies of the Central Powers in World War I.

The demoted Falkenhayn, meanwhile, took control of Germany’s operations against Romania; in this he was joined by another prominent German general, August von Mackensen. By December 1916, Falkenhayn and Mackensen had led their troops to a decisive victory against Romania, overrunning much of the country and occupying the capital city, Bucharest, on December 9, 1916. Though Russian troops entered Romania early the following year, the Russian army was on the verge of collapse; with the Russian Revolution that year, the rise to power of the Bolsheviks, and Russia’s subsequent exit from the war in early 1918, Romania was forced to surrender to the Central Powers at Bucharest that May, having suffered some 335,000 casualties during the course of the war, not including civilian deaths.

According to the Peace of Bucharest, Romania lost land along its coast to Bulgaria, as well as control of the mouth of the Danube River, which the Central Powers commandeered. The Treaty of Versailles in 1919 reversed these losses, however; it also gave Romania control of the long-desired province of Transylvania.