The limited borders shared by Austria-Hungary and Serbia essentially guaranteed that the contests between these two belligerents would be sharp, direct, and dogged. This was because of the topography of the region: The Serbian capital of Belgrade and a portion of Serbian territory could be easily marched by any army in the region, but this would have left open Serbia’s rather nasty Western border, rocky, jagged and tortuous as it law athwart the range of the Dinaric Mountains.
Thus the Austro-Hungarian army had little choice except to guard this Western border by attacking Serbia through it. The Battle of Cer, however, had already demonstrated the perils of forcing a large army through the region, as the Serbians had handily defeated an August invasion of the superior Austro-Hungarian army.
Beginning in early September, the Austro-Hungarians tried again, crossing roughly the same terrain as before en-route to an invasion and attempted defeat of Serbia.
In a certain respect, the invasion was defensive in nature: following the Battle of Cer, the Serbians had been pressured by Russia and France to launch an offensive against Austria-Hungary. Obliging, the Serbs managed to conduct a limited though modestly successful campaign. The Austro-Hungarians reasoned that if they again invaded Serbia, they would force the Serbs to retreat in order to defend their homeland.
The first round of offensives, known collectively as the Battle of Drina, lasted approximately to October 4. Each side suffered about 17,000 casualties, and the Balkan conflict found itself slipping into trench warfare as the Austro-Hungarians successfully advanced into Serbia, but only very slowly.