By 8 September, it 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 that was 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 Romania was occupied by the Central Powers. The capital 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. For that performance, on 9 January 1917, Mackensen was awarded the Grand Cross of the Iron Cross and became one of only five recipients of that honour in the First World War. Since he now wore every Prussian medal, the Kaiser decided to name a battle cruiser after him, which became the first in a new class. Mackensen became the military governor of the large part of Romania (mainly Wallachia) that was occupied by the Central Powers. He proposed making a German prince the King of Romania, but the initiative fell through. His last campaign was an attempt to destroy the Romanian Army, which had been reorganised. During the Battle of Mărăşeşti, both sides took heavy losses, but the Romanian Army emerged victoriously. Mackensen maintained that he had never been defeated in battle, and he surely was the most consistently successful senior general on either side in World War I. By December 1917, the Russian Army had collapsed, and the Romanian Armed Forces were forced to sign the Armistice of Focșani, followed by the Treaty of Bucharest. On 11 November 1918, Germany signed an an armistice with the Allies under which it had to immediately withdraw all German troops in Romania, the Ottoman Empire, Austria-Hungary and the Russian Empire back to German territory and Allies to have access to those countries. Postwar After the armistice, he and the 200,000 men whom he led back home were rounded up. He was arrested by the agents of the pro-Entente Hungarian leader Mihály Károlyi in Budapest. Mackensen