Just to be clear: Serbia held off Austria - Hungary for over a year (summer 1914 - fall 1915), but during that period Bulgaria was neutral.

Some of the reasons Serbia was able to resist the mighty Austrian Empire are:

• the Empire was able to deploy only a part of its army to the Balkans as they had to fight Russians as well. Pretty much only Austrian 5th and 6th armies were used to invade Serbia. Many of the troops were of the Slavic origin (including significant number of Serbs) and had little desire to fight for their German-speaking Emperor.

• Austrians underestimated the fighting ability of the Serbian army and considered the campaign to be mostly a punitive expedition, for Serbia's involvement in the assassination of archduke Franz Ferdinand. Serbia, however had fought two successful wars in 1912 and 1913, and its troops and officers were highly motivated. The lack in modern weaponry and especially ammunition was mitigated by the supplies from the French relatively early in the conflict. Finally, Serbian Supreme command, led by field marshal Putnik, proved to be more than a match for the Austrian general Potiorek.