The Battle of Drina (Serbian: Bitka na Drini) was fought between the Serbian and Austro-Hungarian armies in September 1914, during World War I. The Austro-Hungarians engaged in a significant offensive over the Drina river at the western Serbian border, resulting in numerous skirmishes (the Battle of Mačkov Kamen and the Battle of Gučevo being the heaviest ones). In early October, the Serbian Army was forced to retreat, and later regrouped to fight in the subsequent Battle of Kolubara.
With most of his forces in Bosnia, general Oskar Potiorek decided that the best way to stop the Serbian offensive was to launch another invasion into Serbia to force the Serbs to recall their troops to defend their much smaller homeland.
Field Marshal Putnik ordered a retreat into the surrounding hills and the front settled in a month and a half of trench warfare, which was highly unfavourable to the Serbs, who possessed heavy artillery that was largely obsolete, had short ammunition stocks, limited shell production (having only a single factory producing around 100 shells a day) and also a lack of proper footwear, since the vast majority of infantry wore the traditional (though state-issued) opanaks, while the Austro-Hungarians had soak-proof leather boots. Most of the war material was supplied by the Allies, who were short themselves. In such a situation, Serbian artillery quickly became almost silent, while the Austro-Hungarians steadily increased their fire. Serbian daily casualties reached 100 soldiers from all causes in some divisions.
During the first weeks of trench warfare, the Serbian Užice Army (one strengthened division) and the Montenegrin Sanjak Army (roughly a division) conducted an abortive offensive into Bosnia. In addition, both sides conducted a few local attacks, most of which were soundly defeated.