 Potiorek planned to launch a converging attack across northern and western Serbia ; the 5th Army was to capture Valjevo and envelope the Kolubara River from the north , and the 6th Army was to secure the <unk> plateau and outflank Serbian units on the Kolubara from the south . The capture of the southeastern Serbian city of Niš was Potiorek 's main objective ; Niš had been Serbia 's capital since July and was a crucial transportation hub for its military . It also acted as a clearing house for munitions produced at the arsenal in nearby Kragujevac . The city 's capture would effectively cut Serbia in two and scatter the Serbian Army . 
