 The Austro @-@ Hungarian assault began at Lazarevac , a strategically located town just south of Belgrade whose capture would have given them access to the Mladenovac railway line and the ability to outflank the Serbian forces holding the road to Belgrade . Further south , the Austro @-@ Hungarians attacked the Serbian 1st Army . During this assault , they made the mistake of attacking its stronger right flank and were met with determined Serbian resistance which prevented them from gaining any ground . Military historian David Jordan notes that had the Austro @-@ Hungarians attacked the junction splitting the 1st and Užice armies , they might have been able to split the Serbs down the centre and gotten hold of an unimpeded passage to the Morava River . The Serbian 1st Army was quick to reinforce its left flank , realizing that any subsequent attack against it would be far less easy to repel . 
