Editing Theophilos (emperor) (section)

===Relations with Bulgaria and Serbia===
In 836, following the expiration of the [[Byzantine–Bulgarian Treaty of 815|20-year peace treaty]] between the Empire and Bulgaria, Theophilos ravaged the Bulgarian frontier. The Bulgarians retaliated, and under the leadership of [[Isbul]] they reached [[Adrianople]]. At this time, if not earlier, the Bulgarians annexed [[Philippopolis (Thrace)|Philippopolis]] and its environs.  [[Khan (title)|Khan]] [[Malamir of Bulgaria|Malamir]] died in 836.

The peace between the [[Serbs]], Byzantine ''[[foederati]]'', and the [[Bulgars]] lasted until 839. [[Vlastimir of Serbia]] united several tribes, and Theophilos granted the Serbs independence; Vlastimir acknowledged nominal overlordship of the Emperor. The annexation of western Macedonia by the Bulgars changed the political situation. Malamir or his successor may have seen a threat in the Serb consolidation and opted to subjugate them in the midst of the conquest of Slav lands. Another cause might have been that the Byzantines wanted to divert attention so that they could cope with the Slavic uprising in the [[Peloponnese]], meaning they sent the Serbs to instigate the war. It is thought that the rapid extension of Bulgars over Slavs prompted the Serbs to unite into a state.{{sfn|Bury|1912|p=372}}{{sfn|Kovačević|Jovanović|1893|pp=38–39}}{{sfn|Stanojević|1908|pp=46–47}}

Khan [[Presian I]] (r. 836–852){{sfn|Fine|1991|p=108}} invaded [[Serbia]]n territory in 839 (see [[Bulgarian–Serbian Wars (medieval)|Bulgarian–Serbian Wars]]). The invasion led to a three-year war, in which Vlastimir was victorious;{{sfn|Fine|1991|p=110}} Presian was heavily defeated, made no territorial gains, and lost many of his men. The Serbs had a tactical advantage in the hills,{{sfn|Runciman|1930|p=88}} and the Bulgars were driven out by the [[Medieval Serbian Army|army of Vlastimir]]. The war ended with the death of Theophilos, which released Vlastimir from his obligations to the Byzantine Empire.