After Romania declared war on Austria-Hungary on 15 August 1916, Mackensen was given command of a multinational army, with General Emil von Hell as chief of staff, of Bulgarians, Ottomans, Austro-Hungarians and Germans. They assembled in northern Bulgaria and then advanced into Dobruja. By 8 September, they had taken the two major forts on the right bank of the Danube, the first in a single day by a force that was outnumbered by the besieged, who were overwhelmed by Mackensen's artillery. Then, a German and Austro-Hungarian army group, commanded by Falkenhayn broke into Wallachia through the Vulkan Pass in the Transylvanian Carpathian Mountains while Mackensen crossed the Danube by seizing bridgeheads on the left bank to shield the Austro-Hungarian engineers who built the long pontoon bridge. The Romanian Army and its Russian allies were forced back between those pincers. After three months of war two-thirds of the territory of the Kingdom of Romania was occupied by the Central Powers. The capital city of Romania, Bucharest was captured by the Central Powers on 6 December 1916, on his 67th birthday. He rode in on a white horse and moved into the Romanian royal palace.