Recall, if you will, that the flashpoint from which WWI sprang, its point of ignition, resided in the assassination of the heir to the throne of Austria-Hungary (Franz Ferdinand) by a cabal of Serbian extremists. While other, larger powers of Europe certainly meant to fight elsewhere and against each other (e.g., Russia, Germany, France, Britain), an Austro-Hungarian invasion of Serbia was a forgone conclusion.
Trouble was, the more often it went on campaign, the more the mighty Austro-Hungarian military exposed itself as weak in morale and leadership.
The Austro-Hungarians first tried to punish Serbia at the very beginning of the war, invading in August 1914.
Serbian forces were initially overwhelmed and overawed, but eventually rallied to attack the Austro-Hungarians in and around the mountain passes at the Battle of Cer. The Serbs lost about 16,500 men in their successful attempt to drive the Austro-Hungarians (who themselves had lost about 23,000 men) out of their country.
In early September, the Serbians found themselves under pressure by their allies to invade Austria-Hungary in order to force it to spend troops defending against a Serbian invasion. At first things went well, but when the Austro-Hungarians launched another invasion of Serbia, the Serbians were forced to pull back. The issue of the Battle of Drina eventually settled into trench warfare (in mid October) along a line running roughly from Shabatz to Liubaviyan.
Finally, in late October, the Austro-Hungarians launched a third invasion of Serbia. Again, things went pretty well initially, and the Serbians pulled back much farther into Serbia than they ever had before, regrouping near the Kolubara river amidst a bloody, draining and demoralizing retreat. Nevertheless, the Serbians had fought strategically: they had understood that the region of Northwestern Serbia deeply favored defensive warfare, and so while they had been pushed back, they had taken a large toll on the Austro-Hungarians in the meantime. Even better, they found themselves in relatively good condition to continue fighting, if only they had more supplies and heavy weapons, their own having been abandoned in the retreat to Kolubara.
While the Russians and British sent naught but apologies, the French promised to supply the Serbians with a number of weapons and ammunition. These did not arrive in time to assist the Serbians in resisting the attacks of the Austro-Hungarians, who had appeared at the Kolubara on November 16. These attacks continually pounded the Serbians backwards, but they only retreated tactically, and only after inflicting deep wounds on their oppressors. The high-point came on December 2: The Austro-Hungarians had advanced deep into Serbia, including the recently fallen capital city of Belgrade. Nevertheless, the Austro-Hungarians had greatly extended their supply lines, and had thus been forced to stop their attacks and advances.
Finally, the promised supplies from their allies had arrived, and the Serbians planned a counter-attack late in the day on December 2. The attack aimed at threatening Belgrade and improving the defensive position, but its primary purpose was to improve the now low morale of the Serbian troops. Even aging Serbian King Peter I joined the troops in an effort to inspire them.
The attack caught the Austro-Hungarians completely by surprise: their artillery was far from the front (and thus unable to help), their lines were thin and extended, . . . the Serbians had even caught them hosting a parade down the streets of Belgrade. Though the attacks goals had been limited, they were achieved completely, forcing the Austro-Hungarians back by several kilometers.
The Serbians, now much enthused, continued their attacks in the following weeks. By December 16, they had retaken Belgrade, thus ending the their mini-campaign and ending the battle: neither the Serbians nor the Austro-Hungarians had the strength to do more at that time. Though Austro-Hungarian troops were still in Serbia, the Battle of Kolubara could only be considered a Serbian victory, given that they had accomplished their goal not to throw their enemy out of Serbia or even stop his advance, but rather to systematically bloody him and choose carefully the time and place of counterattack. The Serbians suffered about 132,000 casualties, but the Austro-Hungarians had suffered about 273,000.