Vlastimir succeeded his father, Prosigoj, in c. 830.
 He united the Serbian tribes in the vicinity.
The Serbs were alarmed, and most likely consolidated due to the spreading of the Bulgarian Khanate towards their borders (a rapid conquest of neighbouring Slavs), in self-defence, and possibly sought to cut off the Bulgar expansion to the south (Macedonia).
 Emperor Theophilos (r. 829–842) was recognized as the nominal suzerain (overlord) of the Serbs, and most likely encouraged them to thwart the Bulgars.
The thirty-year-peace treaty between the Byzantines and Bulgars, signed in 815, was still in effect.
According to Constantine VII, the Serbs and Bulgars had lived peacefully as neighbours until the Bulgar invasion in 839 (in the last years of Theophilos).
It is not known what exactly prompted the war, as Porphyrogenitus gives no clear answer; whether it was a result of Serbian-Bulgarian relations, i.e., the Bulgar conquest to the southeast, or a result of the Byzantine-Bulgarian rivalry, in which Serbia was allied with the Byzantines.
According to Porphyrogenitus, the Bulgars wanted to continue their conquest of Slav lands and subjugate the Serbs.
Presian I (r. 836–852) launched an invasion into Serbian territory in 839, which led to a war that lasted for three years, in which the victorious army of Vlastimir expelled Presian from Serbia; Presian lost a large number of his men, and made no territorial gains.
The Serbs had an advantage in the forests and gorges.
The defeat of the Bulgars, who had become one of the greater powers in the 9th century, shows that Serbia was an organized state, fully capable of defending its borders, and possessed a very high military and administrative organization.
It is not known whether Serbia at the time of Vlastimir had a fortification system and developed military structures with clearly defined roles of the župan.
After the victory over the Bulgars, Vlastimir's status rose, and according to Fine he went on to expand to the west, taking Bosnia, and Herzegovina (known as Hum).
In the meantime; Braničevo, Morava, Timok, Vardar and Podrimlje were occupied by the Bulgars.
Vlastimir married off his daughter to Krajina, the son of a local župan of Trebinje, Beloje, in ca.
847/848.
With this marriage, Vlastimir elevated the title of Krajina to archon.
The Belojević family was thus entitled to rule Travunia.

After Vlastimir's death, the rule was divided among his three sons: Mutimir, Strojimir and Gojnik.
The brothers defeated the Bulgars once again in 834–835, also capturing the son of the Bulgarian Khan.
The Serbs and the Bulgarians concluded peace, and the Christianization of the Slavs began; by the 870s the Serbs were baptized and had established the Eparchy of Ras (mentioned in the Fourth Council of Constantinople, 878–880), on the order of Emperor Basil I. Mutimir maintained the communion with the Eastern Church (Constantinople) when Pope John VIII invited him to recognize the jurisdiction of the bishopric of Sirmium.
The Serbs and Bulgarians adopted the Old Slavonic liturgy instead of the Greek.
Sometime after defeating the Bulgarians, Mutimir ousted his brothers, who fled to Bulgaria.
He kept Gopnik's son Petar Gojniković in his court, but he managed to escape to Croatia.
Mutimir ruled until 890, being succeeded by his son Prvoslav.
However, Prvoslav was overthrown by Petar who had returned from his exile in Croatia in c. 892.
Vlastimir succeeded his father, Prosigoj, in c. 830.  He united the Serbian tribes in the vicinity. The Serbs were alarmed, and most likely consolidated due to the spreading of the Bulgarian Khanate towards their borders (a rapid conquest of neighbouring Slavs), in self-defence, and possibly sought to cut off the Bulgar expansion to the south (Macedonia).  Emperor Theophilos (r. 829–842) was recognized as the nominal suzerain (overlord) of the Serbs, and most likely encouraged them to thwart the Bulgars. The thirty-year-peace treaty between the Byzantines and Bulgars, signed in 815, was still in effect. According to Constantine VII, the Serbs and Bulgars had lived peacefully as neighbours until the Bulgar invasion in 839 (in the last years of Theophilos). It is not known what exactly prompted the war, as Porphyrogenitus gives no clear answer; whether it was a result of Serbian-Bulgarian relations, i.e., the Bulgar conquest to the southeast, or a result of the Byzantine-Bulgarian rivalry, in which Serbia was allied with the Byzantines. According to Porphyrogenitus, the Bulgars wanted to continue their conquest of Slav lands and subjugate the Serbs. Presian I (r. 836–852) launched an invasion into Serbian territory in 839, which led to a war that lasted for three years, in which the victorious army of Vlastimir expelled Presian from Serbia; Presian lost a large number of his men, and made no territorial gains. The Serbs had an advantage in the forests and gorges. The defeat of the Bulgars, who had become one of the greater powers in the 9th century, shows that Serbia was an organized state, fully capable of defending its borders, and possessed a very high military and administrative organization. It is not known whether Serbia at the time of Vlastimir had a fortification system and developed military structures with clearly defined roles of the župan. After the victory over the Bulgars, Vlastimir's status rose, and according to Fine he went on to expand to the west, taking Bosnia, and Herzegovina (known as Hum). In the meantime; Braničevo, Morava, Timok, Vardar and Podrimlje were occupied by the Bulgars. Vlastimir married off his daughter to Krajina, the son of a local župan of Trebinje, Beloje, in ca. 847/848. With this marriage, Vlastimir elevated the title of Krajina to archon. The Belojević family was thus entitled to rule Travunia.
After Vlastimir's death, the rule was divided among his three sons: Mutimir, Strojimir and Gojnik. The brothers defeated the Bulgars once again in 834–835, also capturing the son of the Bulgarian Khan. The Serbs and the Bulgarians concluded peace, and the Christianization of the Slavs began; by the 870s the Serbs were baptized and had established the Eparchy of Ras (mentioned in the Fourth Council of Constantinople, 878–880), on the order of Emperor Basil I. Mutimir maintained the communion with the Eastern Church (Constantinople) when Pope John VIII invited him to recognize the jurisdiction of the bishopric of Sirmium. The Serbs and Bulgarians adopted the Old Slavonic liturgy instead of the Greek. Sometime after defeating the Bulgarians, Mutimir ousted his brothers, who fled to Bulgaria. He kept Gopnik's son Petar Gojniković in his court, but he managed to escape to Croatia. Mutimir ruled until 890, being succeeded by his son Prvoslav. However, Prvoslav was overthrown by Petar who had returned from his exile in Croatia in c. 892.
Principality of Serbia (early medieval)