 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 Jagodnja 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 . 
