commander. The reluctant Austro-Hungarian supreme command agreed that the tactful Mackensen was the best choice for commanding the coalition army. Army Group Mackensen (Heeresgruppe Mackensen) was established, containing a new German Eleventh Army, also under his command, and the Austro-Hungarian Fourth Army. As chief of staff, he was assigned Hans von Seeckt, who described Mackensen as an amiable, "hands-on commander with the instincts of a hunter". His army group, which had an overwhelming advantage in artillery, smashed through the Russian lines between Gorlice and Tarnow and then continued eastward. Never giving the Russians time to establish an effective defence, it retook most of eastern Galicia by recapturing Przemyśl and Lemberg. The joint operation was a great victory for the Central Powers, which had advanced 310 km (186 mi), and the Russians pulled out of all of Poland soon afterward. Mackensen was awarded oak leaves to his Pour le Mérite on 3 June 1915 and was promoted to field marshal on 22 June. He also received the Order of the Black Eagle, Prussia's highest-ranking order of knighthood, as well as numerous honorus from other German states and Germany's allies, including the Grand Cross of the Military Order of Max Joseph, the highest military honor of the Kingdom of Bavaria, on 4 June 1915. Serbian Campaign In October 1915, a new Army Group Mackensen (Heeresgruppe Mackensen, which included the German Eleventh Army, the Austro-Hungarian Third Army and the Bulgarian First Army), launched a renewed campaign against Serbia. The campaign crushed effective military resistance in Serbia but failed to destroy the Serbian army, half of which managed to withdraw to Entente-held ports in Albania and, after recuperation and rearmament by the French and the Italians, re-entered fighting on the Macedonian front. When Mackensen returned to Vienna, he was honoured by a dinner and a personal audience with Emperor Franz Joseph and was decorated with the magnificently jeweled Military Merit Cross 1st Class with Diamonds, a unique award for a foreigner. Mackensen appears to have had great respect for the Serbian Army and Serbs generally. Before departing to the Serbian front in 1915, he had spoken to his men: Romanian Campaign 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. It assembled in northern Bulgaria and then advanced into Dobruja.