Setting the stage
On July 23, Austria-Hungary demanded Serbia comply with its March 1909 declaration to the Great Powers to maintain good neighborly relations with Austria-Hungary and issued the July Ultimatum. Serbia responded by mobilizing its army and then responding to the Austro-Hungarian letter accepting point number 10, but cleverly rewording (providing itself with an out), rejecting, or responding disingenuously to the other nine demands. The Austro-Hungarian ambassador rejected the response on the spot and returned to Vienna. Serbian reservists accidentally crossed onto the Austro-Hungarian half of the river at Temes-Kubin and Austro-Hungarian troops fired into the air to warn them off. Exaggerated reports of this incident were used to persuade Emperor Franz-Joseph to give the final go ahead to formally mobilize the southern Austro-Hungarian Army and declare war against Serbia.
Austrian troops executing captured Serbians in 1917.
For complex reasons, the dispute between Austria-Hungary and Serbia escalated into what is now known as World War I, which involved Russia, Germany, France, and United Kingdom. Within a week, Austria-Hungary had to face a war with Russia, which had the largest army in the world at the time. The result was that Serbia became just another front to the massive fight that started to unfold along Austria-Hungary's border with Russia. Serbia had an experienced army, having fought two wars in the last two years, but it was also exhausted and poorly equipped, and the Austro-Hungarians thought that it would fall in less than a month.
Serbia's strategy was to hold on as long as it could and hope the Russians could defeat the main Austro-Hungarian Army. Serbia constantly had to worry about its hostile neighbor to the east, Bulgaria, with which it had fought several wars, most recently in 1913.
The Serbian army at the start of the war was some 180,000 strong, commanded by Marshal (Vojvoda) Radomir Putnik. However he was in poor...