All I know about Serbian war is from the movie "Behind enemy lines"
Unfortunately, I can't think of any major Eastern Front movies in English Long story short:
1) Austro-Hungarians got ****** off that the kingdom that operated in the area wouldn't allow to be occupied by their army as a part of a criminal investigation, so they sent in some armies anyway
2) Once those armies got their ***** kicked, they sent more armies
3) Once those armies got their ***** kicked, they offered to return some territory to the Bulgarians, that they lost in a previous war, so the Bulgarians jumped in
4) Fighting an empire and a well-stocked kingdom was a bit much for the army, the French and the English weren't sending any help, so the army retreated to Greece. (*)
5) The 1/3 of the army that survived steps 1, 2, 3 and 4 got refitted in Greece, licked their wounds, joined the French/British token forces and headed home
6) The joint force kicked some Austro-Hungarian, Turkish and Bulgarian butts and went home
(*) Note that the Austro-Hungarian and Bulgarian "soldiers" really enjoyed killing civilians.. Once you added up the numbers from the war and the war crimes, more than half the men and quite a few women in the country got killed.