 On 13 December , von Frank informed Potiorek that he considered it impossible for Austro @-@ Hungarian forces to remain in Belgrade for much longer . As a result , Potiorek ordered the Austro @-@ Hungarian forces in the city to withdraw . The Austro @-@ Hungarians left Belgrade on 14 and 15 December and retreated back into Austria @-@ Hungary under the cover of their river monitors on the Sava and the Danube . The Serbian Army re @-@ entered Belgrade on 15 December and was in full control of the city by the end of the following day . 
