Serbia is a landlocked South Slavic nation in southeastern Europe. Serbia's origins go back to the seventh century, when Slavs who had migrated into the former Roman provinces of Illyricum, Moesia, and Dalmatia formed the Serbian Principality. The Serbians converted to Orthodox Christianity over the course of the ninth century, then fought a series of wars with the Bulgars before being conquered by the Byzantine Empire in the tenth century.
Byzantine rule ended in the eleventh century and a second Serbian state, the Grand Principality, was founded. The Grand Principality gradually expanded into formerly Byzantine territories until Grand Prince Stefan was crowned King of Serbia in 1217. As the Byzantine Empire weakened in the fourteenth century Serbia expanded, reaching its greatest extent in the middle of the century before suffering political fragmentation and falling to the growing power of the Ottoman Turks. By the middle of the fifteenth century the final remains of the Kingdom of Serbia were conquered by the Ottoman Empire.
The Serbs remained under the rule of the Ottoman Empire at the time of the North American Rebellion in the 1770s. However, the Ottoman Empire had been suffering a steady decline in power since the failed Siege of Vienna in 1683, and sometime in the nineteenth century the Serbs established an independent state. This reborn Serbian state was allied with France in the early twentieth century, most likely in response to an alliance between the Ottoman Empire and the Germanic Confederation.
Sobel makes no mention of Serbia in the Global War, but it is possible that Serbia joined Germany in its conquest of the Ottoman Empire in 1939-40. If so, then Serbia presumably remains allied with the German Empire in the era of the War Without War.
Serbia does not have an entry in Sobel's index.
In For All Nails Serbia was reconquered by the Ottoman Turks after the Hundred Day War and then incorporated into Croatia during the Global War.