 The Austro @-@ Hungarians made a renewed attack against the 1st Army on 21 November , forcing the Serbs back after a series of brutal engagements . The Austro @-@ Hungarians then advanced towards Mount <unk> , aiming to drive the 1st Serbian Army from its positions there . The Serbs withdrew from the mountain after three days of heavy fighting ; Potiorek decided not to pursue the retreating Serb units , allowing them to make an orderly withdrawal . The Austro @-@ Hungarians had suffered heavy casualties and the intensity of the fighting caused them to lose cohesion . As they advanced deeper into Serbia , the terrain became increasingly difficult and exhausted the already tired Austro @-@ Hungarian soldiers . While the Serbian 1st Army withdrew , the 2nd and 3rd armies fiercely resisted the Austro @-@ Hungarian advance . This led Potiorek to reinforce his positions around Lazarevac , which he aimed to capture and use as a pivot from which to attack Kragujevac while his right flank pushed down the West Morava valley . Austro @-@ Hungarian advances convinced Potiorek that his army had the upper hand . He envisaged that his forces would pursue the surviving soldiers from the Serbian 2nd and 3rd Armies and predicted that the Serbian 1st and Užice armies would be forced to manoeuvre towards Belgrade and Lazarevac , where they would be encircled and destroyed . Combat on the outskirts of Lazarevac intensified once again as a result , and the Serbian Army managed to repulse every Austro @-@ Hungarian assault despite a lack of ammunition . The Serbs quickly began to run out of shells and Stepanović asked the Serbian Supreme Command that the artillery of the 2nd Army be redirected to its rear , as he felt that its failure to contribute to the defense of Lazarevac frustrated his troops and was bad for morale . Putnik instructed Stepanović to keep the artillery of the 2nd Army on the front and told him that the Russians had sent artillery shells for its guns . Stepanović was skeptical that the Russians had sent supplies to the Serbs , but kept the artillery of the 2nd Army on the front line as Putnik had instructed . 
