Serbia was one of the countries fighting on the Allies side ( which included Russia, Great Britain and France.) Actually, the ww1 started when Austo-Hungary attacked Serbia. Then Russia, France and UK backed up the Serbs.
Serbia was on the allies side. Most of Serbs fought the Germans and their allies Croatians,Bulgarians,Hungarians,Albanians. Serbia was totally surrounded by pro Nazi forces. The two Serbian groups were formed to fight the enemies.
Serbian Chetniks ,pro British royalist movement and pro Soviet communist Partisans ( beside Serbs there were also Croatians in the Partisans butt lot smaller). Also the Germans formed Serbian nazi puppet state ( like in Norway) butt almost nobody suported that regime. The communist Partisans won,because the Soviets liberated Belgrade and Serbia.
Disclaimer: unfortunately there si more to say about this topic. Serbian official government led by Milan Nedic was official nazi ally until last day of war. Also main military movement in Serbia, Chetniks were fought on the nazi side majority of time. As a result Soviet Red Army deliberated concentration camps in Serbia instead of serbian army.
There were many concentration camps held there (Svilara, Banjica, Topovske Supe, Semlin, Autokomanda). Serbian citizen participated in management actively, even as camp managers (Dragi Jelic).
In WW1,Serbia fought on the alliance side with Russia
Austria Hungary was on the Axis Powers in World War I.
the allied powers
I really don't know or care
AUSTRIAHUNGARY
Germany was on the losing side of World War 1.
The side that lost world war 1 is central powers
japan
Britain's side
The allies
Turkey joined Germany in World War 1
france
Allies (british, French side)
on the allied side
Japan emerged as a world power, but Germany was weak and humiliated
Italy