Serbia

Rulers:
King Peter I of Serbia, Petar Karađorđević

Military commanders:
Marshal (Vojvoda) Radomir Putnik

Pros:
Well organised army (Was capable of a full retreat and loss of most of the nation, and started fighting again 6 months later)
Well Commanded

Cons:
Poorly equiped, relied on Allied equipment (For example ran low on shells fairly early on).
Hated by Bulgaria
Comparativly low population

Allies:
The Entente (Mainly Russia)