Strymon ( theme ) The Theme of Strymon ( ) was a Byzantine military - civilian province ( theme ) located in modern Greek Macedonia , with the city of Serres as its capital . Founded probably by the mid - to - late 9th century , its history as an administrative history was chequered , being variously split up and / or united with neighbouring themes . Location . The theme covered the region between the Strymon and Nestos rivers , between the Rhodope mountains and the Aegean Sea . The area was strategically important . Not only did the theme control the exits to the mountain passes from the Slav - dominated interior of the Balkans into the coastal plains of Macedonia , but it was transversed by the great " Via Egnatia " highway , which linked Byzantine - controlled Thrace with Thessalonica , the Empire ' s second - largest city . The region was peopled predominantly with Slavs from the late 7th century on , and retained a significant Slavic population at least until the 11th century . Its main cities were Serres , Philippi , Christoupolis and Chrysopolis , while it may also initially have included the cities of Xanthi and Mosynopolis east of the Strymon . History . In the 8th century , Strymon was a " kleisoura " of Macedonia . The exact date of its establishment as an independent theme is unknown , but it probably dates to the first half of the 9th century . A passage in Theophanes the Confessor dated to 809 may imply its existence already at that date , but its governor is not included in the list of offices known as the " Taktikon Uspensky " of c . 842 . The " strategos " of Strymon first appears in the 899 " Kletorologion " , although a series of seals naming both " archon tes " and " strategoi " of Strymon are known from the second quarter of the 9th century . In addition , the bishop of Serres was elevated to an archbishop at about the same time , a possible indication of the establishment of a thematic capital there . Several authors like the French Byzantinist Paul Lemerle support its creation in the late 840s , during Theoktistos ' s anti - Slavic campaigns , but historian Warren Treadgold considers it to have been become a full theme in c . 896 , to counter the threat of the Bulgarian tsar Symeon I ( r . 893 – 927 ) . In the late 10th century , the theme was divided in two parts : Strymon proper , also known as Chryseuba or Chrysaba ( Greek : Χρυσεύβα / Χρυσάβα , according to the Greek scholar Nikolaos Oikonomides a Hellenized form of " Krushevo " , modern Achladochori ) , and the theme of New Strymon ( Greek : Νέος Στρυμών ) . The latter is known only through the " Escorial Taktikon " of c . 975 . Oikonomides identifies it either with the portion of the old theme east of the Nestos , which was later raised to a separate theme as Boleron ( Greek : Βολερόν ) , or with a northern portion along the upper Strymon , possibly acquired after Emperor John I Tzimiskes ' s ( r . 969 – 976 ) conquest of Bulgaria in 971 . Towards the end of the 10th century , the theme of Strymon appears to have been united with that of Thessalonica and perhaps also Drougoubiteia , while in the 11th century it appears united with Boleron . The theme continued in existence until the dissolution of the Byzantine Empire by the Fourth Crusade ( 1204 ) , when it became part of the short - lived Latin Kingdom of Thessalonica . In 1246 , after the Nicaean emperor John III Vatatzes ( r . 1221 – 1254 ) conquered Macedonia , the theme was re - established as a separate province . In the 14th century , however , it again appears as combined with other provinces such as the themes of Boleron and Thessalonica or as the theme of " Serres and Strymon " . It was permanently dissolved after the region ' s conquest by the Serbian Empire in the 1340s , during a Byzantine civil war .