 On 8 December , the Austro @-@ Hungarians fell back against Užice and Valjevo . The Serbs anticipated that their opponents would entrench themselves and attempt to block the Serbian Army 's advance , but the Austro @-@ Hungarians had failed to construct any defensive networks and , as such , were in no position to block the Serbian offensive . The Austro @-@ Hungarians had ensured that Valjevo 's defenses were fortified and had laid down artillery plans for the town 's defense , but their lack of prior preparation meant that the hills surrounding the town were devoid of any significant defensive positions . The Serbs exploited this weakness by manoeuvring around the hills and encircling the Austro @-@ Hungarians , suffering minimal casualties . The Serbian 3rd Army then broke through the defenses of the 6th Army at Mount Suvobor and stormed Valjevo . In Niš , the Bulgarian ambassador to Serbia reported : " The most improbable news from the battleground , sweet to the Serb ear , has been going around since this morning . " He wrote that , in the last three to four days , the Serbian Army had captured one Austro @-@ Hungarian General , 49 officers and more than 20 @,@ 000 troops , as well as 40 cannon and " huge quantities of war matériel " . By 9 December , the Austro @-@ Hungarian counter @-@ offensive around Belgrade lost its momentum and the Austro @-@ Hungarians began to retreat back towards the city centre . One Austro @-@ Hungarian soldier wrote : " We could not have imagined that the Serbs were on our heels , after all we had recently been victorious . " On 10 December , the Serbian Army captured the lower reaches of the Drina , forcing the majority of surviving Austro @-@ Hungarian troops to flee across the river . They did not stop until they had crossed the Sava and the Danube and entered the Banat . Very few Austro @-@ Hungarian soldiers made it back into Bosnia . 
