 All of the valleys of northwestern Serbia were swamped by constant rainfall . The mountains had been covered in snow since early October . Acknowledging the opportunity that such conditions presented , Putnik told his closest advisors : " All my strategy consists in placing the ' Serbian national mud ' between the enemy 's fighting line and his supplies . " On 31 October , von Frank 's 5th Army pushed down into the region between the Sava and Drina rivers while Potiorek 's 6th Army drove west across the Drina and into the <unk> plateau . Austria @-@ Hungary 's third invasion of Serbia commenced on 6 November 1914 , with intense artillery fire strafing a series of Serbian border towns . On 7 November , the Austro @-@ Hungarian 5th and 6th armies attacked across the Drina . Despite being outnumbered and in desperate need of ammunition , the Serbian Army offered fierce resistance but was forced into strategic withdrawal . The 3rd Army fell back against a road by the Jadar River in an effort to block the Austro @-@ Hungarian advance towards Valjevo , while the 1st Army retreated southward into the Serbian interior and the Užice Army managed to prevent the Austro @-@ Hungarians from crossing the Drina . 
