 The Serbs beat back an Austro @-@ Hungarian invasion in August , at the Battle of Cer . It marked the first Allied victory over the Central Powers in World War I. Potiorek was humiliated by the defeat and was determined to resume the assault against the Serbs . He was given permission in September to launch another invasion of Serbia provided that he " [ did not ] risk anything that might lead to a further fiasco . " Under pressure from the Russians to launch their own offensive and keep as many Austro @-@ Hungarian troops as possible away from the Eastern Front , the Serbs invaded Bosnia in September with the help of Chetnik irregulars but were repulsed after a month of fighting in what came to be known as the Battle of the Drina . Bojović was wounded during the battle and was replaced by Živojin Mišić as commander of the Serbian 1st Army . 
