Elements of the Austro-Hungarian army had began an invasion of Serbia in the early days of August. To briefly summarize some of the most important of the numerous influences which motivated this action:
-The Balkans (The Balkan Peninsula just west of Istanbul) had become very unstable by 1914. The Ottoman Turks had originally dominated the area, but centuries of weakness had created a power vacuum in the region, leading to numerous land grabs by neighboring nations.
-A 1908 annexation of Bosnia by Austria-Hungary enraged the Serbians. They had long considered Bosnia, with her significant proportions of Orthodox, Serbian population to be destined for incorporation into Serbia. They called upon their Orthodox brother nearby (Russia), but to no avail. The Russians had been recently humiliated in the Russo-Japanese war, and could not risk further exposing their weaknesses.
-Balkan Wars in 1912 and 1913 occurred so quickly that Austria-Hungary was unable to take part in the power grab. Worse, their ally Germany felt that these regional conflicts were simply too petty to justify their support.
-The assassination of the imperial Austrian heir, Franz Ferdinand, brought tensions to a head: Austria-Hungary felt that they finally had an excuse to punish Serbia that all the world would understand, and Serbia hoped to damaged Austro-Hungarian prestige, at the very least. Unfortunately for the rest of Europe, their allies (Germany with Austria-Hungary, Russia and therefore France with Serbia) did not step away as they had in 1908, 1912 and 1913, escalating the conflict into one which embroiled the entire continent.
OK, back to the invasion.
Germany begged Austria-Hungary to hold back significant elements of its army of more than 3,000,000 troops, for Germany anticipated the need for Austria-Hungary to help resist a flood of Russian troops from the East. Franz Conrad von Hötzendorf, the Austro-Hungarian field marshal, reluctantly agreed, invading Serbia with approximately 270,000 troops.
In mid-August, approximately 200,000 of these had set up camp on or near the slopes of Mt. Cer. A Serbian army of about 180,000 troops attempted to dislodge them.
The Serbians were fewer in number, comparatively poorly equipped, and enjoyed far fewer support units (such as machine guns and artillery). A battle where both sides traded attacks, defenses and counter-attacks, perhaps the only way to explain the almost total Serbian success is a remarkable weakness of morale among the Austro-Hungarian troops: When a Serbian attack failed, they generally regrouped quickly, and when they fell back, they managed to maintain some cohesion. Austro-Hungarian troops, however, seemingly lost heart easily, being especially vulnerable to counter-attacks after one of their attacks had failed . . . they often retreated in disorder at this battle.
Most of the battle was over by the end of the day, August 19th. Austro-Hungarian troops were turning away from the battle, and the Serbian army, stayed on its heels, eager to chase them back across the Serbian border.
Germany had made certain to support Austria-Hungary in its 1914 dispute against Serbia, whatever the risk, whatever the cost, because she needed to maintain her only alliance in central Europe. Unfortunately for Germany, Austria-Hungary would over and over manifest immense weaknesses, especially in the morale of its armies. Being allied to Austria-Hungary, as the war went on, increasingly became like being chained to a corpse.