Around 23:00 on 15 August, elements of the Serbian 1st Combined Division encountered outposts set up by the invading Austro-Hungarian army on the slopes of Cer Mountain and fighting erupted. The Austro-Hungarian positions were lightly held, and their defenders were driven back away from the mountain. By midnight, fierce clashes between the Austro-Hungarians and the Serbs were underway and chaos ensued in the darkness. By the morning of 16 August, the Serbians had seized the Divača Range and dislodged the Austro-Hungarians from their positions in the village of Borino Selo. The Austro-Hungarians, who had suffered heavy casualties during the fighting, retreated in some disorder. As the day progressed, the Serbs drove the 21st Infantry Division off the slopes of Cer to prevent it from linking with the 2nd Army in Šabac.