Romania joined the Allies in World War I in August 1916 when its forces invaded Transylvania across the border in the Carpathian Mountains. The Romanian forces quickly defeated the small number of Austro-Hungarian forces based in the border area and started their advance into Austro-Hungarian territory, but were soon halted. Meanwhile, a Central Powers force comprising Bulgarian, German and Turkish troops and led by August von Mackensen entered Dobruja in southeastern Romania.