First Army ( Serbia ) The Serbian First Army ( Српска Прва Армија / Srpska Prva Armija ) was a Serbian field army that fought during World War I . History . Early World War I . Following the Austro - Hungarian invasion of Serbia , the First Army was put under the command of General Petar Bojović . It acted as a strategic reserve in the area of Aranđelovac during the Battle of Cer , but most of its divisions were sent to support the Second and Third armies actively engaged in the battle . The army conducted a successful crossing of Sava and performed an offensive into Syrmia ( then part of Austria - Hungary ) but was recalled when Second invasion of Serbia ( also known as the Battle of Drina ) began . The army had the decisive role in the battle conducting a strong counterattack against the Austro - Hungarian 6th army . It was engaged in some of the fiercest fights of the whole Serbian theatre at Mackov kamen , which ended in a bloody stalemate . After a month - long period of trench warfare , in November 1914 , the Austro - Hungarian Army began the third invasion of Serbia ( also known as the Battle of Kolubara ) . During the defensive part of this battle this army was in the most difficult situation due to heavy casualties from the Battle of Drina and acute lack of artillery ammunition . It was at this point that general Živojin Mišić , who was previously the Aide of Commander in Chief of Serbian Army , ( Vojvoda / Duke - equalent of Field Marshal Radomir Putnik ) , was made commander of this army , as Bojović was wounded . He restored morale and discipline ( which had started to waver ) in the army , by insisting on a deeper withdrawal before the Austro - Hungarians , all the way to Gornji Milanovac . In December 1914 , after receiving a badly needed supply of 75mm artillery ammunition , the whole Serbian army launched a counterattack . The First Army played the decisive role in this having affected the breakthrough against the 16 . corps of Austro - Hungarian 6th army . It subsequently pursued that army all the way to the Drina , while the other two armies were engaged in expulsing the Austro - Hungarian 5th army . After the battle , Mišić promoted to Vojvoda . When the fourth , combined German , Austro - Hungarian , and Bulgarian invasion force attacked Serbia in 1915 , the First Army was tasked with defence of the western border of Serbia ( along the river Drina ) . It faced mostly units of Austro - Hungarian Third Army and was mostly under less pressure then other Serbian units facing Germans or Bulgarians . Perhaps as a cousequence of this Mišić was in favor of halting and making a final stand against the Central Powers forces when the High Command ordered retreat across Montenegro and Albania to Italian - held Durazzo and Valona ( at the time Serbian Army was defending itself in Kosovo from three sides and was in danger of becoming surrounded ) . However , he was overruled by other army commanders at their meetings at Peć , who decided to carry on the order of High Command . Retreat . The First Army took part in the epic retreat of the Serbian Army through the Albanian mountains during the winter of 1915 and 1916 , during which it suffered heavy casualties due to hunger , frostbite , attack from Albanian bands ( Albania officially supported the Entente ) and exhaustion . From the Italian - held ports the Serbian Army was transported by Allied ships to Corfu , and a smaller part to French - held Bizerte to recuperate and reorganise . During the retreat of the Serbian army ( along with civilians ) a few Serbian soldiers would be assigned to " guard " large groups of civilians of 300 to 500 persons . Vardar Offensive . Mišić was recalled to command the First Army in September 1916 initially at Monastir and then at Thessaloniki . The First Army ( again commanded by Bojović ) took part in the Allied Vardar Offensive of 1918 that effectively knocked Bulgaria out of the war . During this offensive , this army advanced far ahead of the Allies and performed a maneuver near Niš which broke the last serious German attempt to halt the offensive by superior numbers . For his merit in this feat , Gen . Bojović was promoted to Vojvoda . This army liberated Belgrade , and subsequently crossed into Vojvodina and advanced as far as present - day border between Serbia and Hungary by the time of the Armistice . Army composition . The numerical strength and unit composition of all Serbian armies varied greatly during the war not only because of the heavy casualties , but also because of the very flexibile strategy employed by the Serbian High command , with armies often trading regiments and even divisions if needed . Serbian unit organisation did not include brigade s and corps . The regiments directly formed the divisions ( mostly four ) , and divisions formed the armies .