Banat Hypothesis Z Dobruja Giurgiu attacked Transylvania German 9th Army Bucharest surrendered Southern Dobruja Russia Bulgaria Central Powers Transylvania offensive Romania Hungarian
In September 1914, Russia had acknowledged Romanian rights to Austro-Hungarian territories including Transylvania and Banat, whose acquisition had widespread popular support,  and Russian success against Austria led Romania to join the Entente in the August 1916 Treaty of Bucharest. Under the strategic plan known as Hypothesis Z, the Romanian army planned an offensive into Transylvania, while defending Southern Dobruja and Giurgiu against a possible Bulgarian counterattack. On 27 August 1916, they attacked Transylvania and occupied substantial parts of the province before being driven back by the recently formed German 9th Army, led by former Chief of Staff Falkenhayn. A combined German-Bulgarian-Turkish offensive captured Dobruja and Giurgiu, although the bulk of the Romanian army managed to escape encirclement and retreated to Bucharest, which surrendered to the Central Powers on 6 December 1916.
