The Bulgarian rule over Serbia lasted only three years.
After Symeon died, Časlav Klonimirović (927- c. 960s) led Serb refugees back to Serbia.
He secured the allegiance of the Dalmatian duchies and ended Bulgarian rule in central Serbia.
After Tomislav’s death, Croatia was in near-anarchy as his sons vied for sole rule, so Caslav was able to extend his domain north to the Vrbas river (gaining the allegiance of the chiefs of the various Bosnian župas).

During this apogee of Serbian power, Christianity and culture penetrated Serbia, as the Serb prince lived in peaceful and cordial relations with the Byzantines.
However, strong as it had grown to be, Serbia’s power (as in other early Slavic states) was only as strong as its ruler.
There was no centralized rule, but instead a confederacy of Slavic principalities.
The existence of the unified Grand Principality was dependent on the allegiance of the lesser princes to Caslav.
When he died defending Bosnia against Magyar incursions sometime between 950 and 960, the coalition disintegrated.

After this, there is a gap in the history of hinterland Serbia--in western sources, Rascia--as it was annexed by the Byzantine Empire and the Bulgarian Empire.
The dynasty continued to rule the maritime regions, and, in the 990s, Jovan Vladimir (Vlastimirović) rose as the most powerful Serbian prince, ruling over present-day Montenegro, eastern Herzegovina, and Koplik in Albania.
 This state became known as Duklja, after the ancient Roman town of Doclea.
However, by 997, it had been conquered and made subject to Bulgaria again by tsar Samuel.

When the Byzantines finally defeated the Bulgarians, they regained control over most of the Balkans for the first time in four centuries.
Serbian lands were governed by a strategos presiding over the Theme of Sirmium.
However, local Serbian princes continued to reign as suzerains to the Byzantines, maintaining total autonomy over their lands, such as in Rascia, while only nominally being Byzantine vassals.
Forts were maintained in Belgrade, Sirmium, Niš and Braničevo.
These were, for the most part, in the hands of local nobility, which often revolted against Byzantine rule.
The Bulgarian rule over Serbia lasted only three years. After Symeon died, Časlav Klonimirović (927- c. 960s) led Serb refugees back to Serbia. He secured the allegiance of the Dalmatian duchies and ended Bulgarian rule in central Serbia. After Tomislav’s death, Croatia was in near-anarchy as his sons vied for sole rule, so Caslav was able to extend his domain north to the Vrbas river (gaining the allegiance of the chiefs of the various Bosnian župas).
During this apogee of Serbian power, Christianity and culture penetrated Serbia, as the Serb prince lived in peaceful and cordial relations with the Byzantines. However, strong as it had grown to be, Serbia’s power (as in other early Slavic states) was only as strong as its ruler. There was no centralized rule, but instead a confederacy of Slavic principalities. The existence of the unified Grand Principality was dependent on the allegiance of the lesser princes to Caslav. When he died defending Bosnia against Magyar incursions sometime between 950 and 960, the coalition disintegrated.
After this, there is a gap in the history of hinterland Serbia--in western sources, Rascia--as it was annexed by the Byzantine Empire and the Bulgarian Empire. The dynasty continued to rule the maritime regions, and, in the 990s, Jovan Vladimir (Vlastimirović) rose as the most powerful Serbian prince, ruling over present-day Montenegro, eastern Herzegovina, and Koplik in Albania.  This state became known as Duklja, after the ancient Roman town of Doclea. However, by 997, it had been conquered and made subject to Bulgaria again by tsar Samuel.
When the Byzantines finally defeated the Bulgarians, they regained control over most of the Balkans for the first time in four centuries. Serbian lands were governed by a strategos presiding over the Theme of Sirmium. However, local Serbian princes continued to reign as suzerains to the Byzantines, maintaining total autonomy over their lands, such as in Rascia, while only nominally being Byzantine vassals. Forts were maintained in Belgrade, Sirmium, Niš and Braničevo. These were, for the most part, in the hands of local nobility, which often revolted against Byzantine rule.
Principality of Serbia (early medieval)