How did Serbia defeat the Austro-Hungarian army?

In 1914 Austria-Hungary was a large nation. It controlled what is today Austria, Hungary, The Czech Republic, Slovakia, Slovenia, Croatia and Bosnia-Herzegovina. In addition it controlled parts of what is now Italy, Serbia, Romania, Poland and Ukraine. In 1914 the Austro-Hungarian empire boasted a population of about 52 million (to put that in perspective in 1914 the United States had a population of about 99 million, France had a population of about 40 million and the Russian Empire had a population of about 167 million.      

Serbia had different borders than today but was roughly the same size and had a population of about 4.5 million, less than a tenth of Austria-Hungary.

Austria-Hungary was a much larger and more powerful nation than Serbia. On the surface it would seem that a rapid Serbian defeat would be inevitable. But there were several issues that tilted the war in Serbia's favor in 1914.

First the war was fought on Serbian territory so the Serbian army was more familiar with terrain that battles were fought on. But more importantly the Serbian army had fought in the 1st and 2nd Balkan Wars (from 1911 to 1913). Most of the Serbian soldiers, officers and generals had wartime experience, the Austro-Hungarian army did not.

But more importantly the Austro-Hungarian army had severe internal issues. About a quarter of its troops were illiterate (which made reading maps difficult and limited the usefulness of written orders). This literacy rate was not shocking for the time in eastern Europe, but it did make managing a modern military difficult. More damaging than the issue of literacy was the issue of language. In Austria-Hungary the official languages were German and Hungarian, but there were a total of 11 officially recognized languages spoken, in addition to dozens of other languages and dialects that were not officially recognized (if you consider the list of modern nations that Austria-Hungary covered you can see how this would be). Many Austro-Hungarian troops fought with fellow soldiers that did not share a common native language. It was common for Austro-Hungarian officers to know how to issue basic commands in all 11 officially recognized languages, which may have worked managing an army in peace time, but in a time of war it had severe consequences.  The Austria-Hungarian army did take steps to try to recertify this problem. It developed an artificial language called "Army Slavic." It was a language that consisted of about 80 words that all members of the Austro-Hungarian army were expected to learn (think basic words like go, stop, left and right). Use of the language was functional but it did not lend itself to a highly efficient military action.

The Serbian army was not without its flaws. Serbia's equipment was not a modern as their Austria-Hungarian opponents. Shortages of supplies and ammunition were common and the Balkan Wars that had gained the Serbian army valuable experience also cost them the lives of many soldiers. The small population of Serbia could never hope to raise an army as large as the Austro-Hungarian army and the casualties suffered from 1911 to 1913 only exacerbated the problem.

In one way the Austro-Hungarian and Serbian armies had something in common. At the start of the war both armies had very high moral. Serbians rallied to protect their homeland and the multicultural Austro-Hungarian Empire, which had been plagued with internal strife, was unified by a desire to avenge the death of Archduke Ferdinand and defeat Serbia, who in the minds of most in Austro-Hungarian people, had threatened their nation and started the war.

The stunning Serbian victories early in the war would fail keep Serbia safe and would only underscore the nature of the industrial warfare that would define World War One. As the war dragged on, victory would not come to the nation with the most brilliant strategies or bravest soldiers, but to the nation that could continue to funnel lives and equipment into the unfillable sieve the war would become. World War One would not end as one set of armies bested an another on the battlefield. It would end as nations collapsed under the strain of industrial war.