I dunno where you got that information? Well, according to the documentory i just watched, Austrias initial invasion of Serbia was a dismal failure once they met the main serbian army fortified in the high ground.
Only when they had German and Bulgarian (well one of those countries that the name starts with 'B') help later on in the war, did they succeed.
However, I have no reason to insist that documentory is correct and you are wrong, after all, i didn't make the documentory, i just watched it!
Okay... I don't know if I saw the 1-hour television special you saw, but I do have a Major in History that took me several years of college to obtain, plus a backlog of over 30 years of reading history privately... I might know one or two things about the first World War compared to what you learned in an hour or two of the boob-tube... Obviously you seem to (wrongly) think the following:
1) Serbia beat Austria's initial invasion in a vacuum outside all outside interference. The fact Austria was also at war with Russia had no impact what-so-ever on Austria's ability to bring war to Serbia.
2) Serbia brushed-off Austria's assault like it was nothing, taking little to no casualties.
3) You assume Serbia could only be defeated when Austria gained outside support from tiny
Bulgaria, but you do not assume the opposite, that Serbia without
Russia's 3,115,000 men in the front lines when war was declared on Austria and shared a common border with them did not help Serbia. Austria diverted huge resources to counter the Russians, but that doesn't matter, I suppose.
In fact the Serbian Army was decimated towards the end of the war, falling from about 420,000 at its peak to about 100,000 at the moment of liberation (after it was conquered). The Kingdom of Serbia lost 725,000 inhabitants during the war (both army and civilian losses), which represented over 17% of its overall population.
You had stated in your initial hypothesis based on a single documentary that Serbia alone (population 4.7 million), in a vacuum could defeat the Austro-Hungarian Empire (57 million) without either nation having outside influence. I state, with a lot of history to back me up, that's a very preposterous assumption.