It's a tragic story really. We won the battles of Cer and Kolubara, but then the rest of our neighbors decided to help by gifting us a knife in the back (which was nothing new), so the army went on the great retreat through Albania. A lot of people died from Typhus and the cold, also the Albanian tribes that were picking off any stragglers. Our great western allies waited for us in ships on the shore of Albania (under the threat of Russian empire that if they don't show up to transfer us to Greece they would retreat from the war). From there we were transferred to the island of Corfu where a lot more people died. After they recovered they went back to help the allies.
Percentage wise Serbia had the largest losses in WW1.