On 19 August, the morale of the Austro-Hungarians collapsed and thousands of soldiers retreated into Austria-Hungary with many of them drowning in the Drina River as they fled in panic.

[8] In 1908, Austria-Hungary formally annexed Bosnia-Herzegovina—a territory with a large Serb population that it had been granted the right to govern, supposedly as a temporary measure, by the Congress of Berlin in 1878.

With no promise of Russian support in the event of war, the Serbian government decided against pursuing the matter militarily.

[10] Count Franz Conrad von Hötzendorf boasted that it would take Austro-Hungary only three months to defeat Serbia should war erupt between the two nations.

[11] With Bosnia-Herzegovina firmly in Austro-Hungarian hands, Serbia and several other Balkan states turned to force the Ottoman Empire from southeastern Europe.

[10] On 28 June 1914, Bosnian Serb student Gavrilo Princip assassinated Archduke Franz Ferdinand of Austria in Sarajevo.

[13] The Austro-Hungarians declared war on Serbia on 28 July and that same day the Serbs destroyed all bridges on the Sava and Danube rivers in order to prevent Austria-Hungary from using them during any future invasion.

This was because a large portion of the Austro-Hungarian 2nd Army had moved to the Russian Front, reducing the number of troops involved in the initial stages of the invasion to approximately 200,000.

Few recruits had been gained from the newly acquired territories, and the Serbian army had been stretched by the need to garrison them against Albanian insurgents and the threat of Bulgarian attack.

To compound matters, the Serbian army was dangerously short of artillery and had only just begun to replenish its ammunition stocks.

[20] Austro-Hungarian forces assigned to the invasion were placed under the command of General Oskar Potiorek, who had been responsible for the security detail of Archduke Franz Ferdinand in Sarajevo.

[23] Generals Petar Bojović, Stepa Stepanović and Pavle Jurišić Šturm commanded the 1st, 2nd and 3rd Serbian Armies, respectively.

[24] From 29 July to 11 August, the Austro-Hungarian army launched a series of artillery attacks in northern and northwestern Serbia and subsequently managed to exploit the bombardments by constructing a system of pontoon bridges across the Sava and Drina rivers.

Putnik thereby re-ordered the Serbian army to fall back on a traditional line of defence as he grouped the bulk of his forces in Šumadija, from where they could rapidly move either north or west.

[25] Belgrade, Smederevo and Veliko Gradište continued to be subjected to more vigorous artillery bombardments, and several attempts to cross the Danube resulted in heavy Austro-Hungarian losses.

[3] The forward battalion had advanced during the night towards the Trojan peak, and when we made it to Parlog the shower began, followed by volcanic thunder and sheet lightning.

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.

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.

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.

[29] In the early morning of 18 August, the Austro-Hungarians launched another attack, with the intention of pushing the 1st Šumadija Division off the Šabac bridgehead to allow the 5th Army to advance.

[5] Elsewhere, the Serbian 2nd Army's counter-offensive continued along the Cer and Iverak, with the 1st Combined Division attacking the village of Rašulijača and coming under severe pressure at Kosanin Grad.

The village of Velika Glava fell to the Serbs before midday, and by late afternoon, the Rajin Grob ridge had been retaken.

Putnik then notified King Peter in a telegram, saying "The main enemy has been defeated in Jadar and on Mount Cer, and our troops are in hot pursuit.

[31] Nevertheless, the number of fatalities suffered by both sides heralded the massive cost in human lives of the First World War.

[37]Although they succeeded in repelling the Austro-Hungarian attack, the Serbs used up much of their ammunition during the battle, needing 6.5 million cartridges and 35,000 shells to prevail.

[2] The first aerial dogfight of the war occurred during the battle, when Serbian aviator Miodrag Tomić encountered an Austro-Hungarian plane while performing a reconnaissance mission over enemy positions.

Certain cultural circles in Italy advocated entering the war on the Allied side, citing Serbian and Montenegrin battlefield successes.