Latins at Plovdiv. Stefan went as far as to become a blood brother of Strez in a ceremony which Stefan was hoping would secure Strez's loyalty. In 1208, Strez headed a Serbian force which seized much of the Vardar valley from Bulgaria. He established himself as a Serbian vassal at the Prosek fortress (near modern Demir Kapija), formerly the capital of another Bulgarian separatist, Dobromir Chrysos. By 1209, Strez's realm spread over much of Macedonia, from the Struma valley in the east, where he bordered the lands controlled by Boril, to Bitola and perhaps Ohrid in the west, and from Skopje in the north to Veria in the south. While Strez quickly gained the support of the local Bulgarian population and possibly inherited the remaining administration from Boril's rule, Serbian units nevertheless remained in his domains, either to guarantee his loyalty or with the intent to oust him and annex his lands. Bulgarian vassal Alexius Slav's marriage to the daughter of Latin Emperor Henry of Flanders in 1209 was potentially a great danger to Boril, who could be facing their joint forces. Fearing such a coalition, Boril approached Strez, who was at the time gaining more power and was close to complete independence from Serbia. Strez agreed to a union with his former enemy, though only after Boril reconfirmed his complete autonomy. Strez eliminated the remaining Serbian troops in his lands in an act that the Serbs saw as devil-inspired treason. It is not impossible that Boril persuaded Strez through military action, though it is more likely that the union was achieved through negotiations. In the same year, Strez and Boril had come to peace with Michael I Komnenos Doukas, the ruler of Epirus. In late 1209, Strez and Michael may have attempted a joint campaign against Thessaloniki, as they both lost lands to the Latins in what was likely a retaliation raid in late 1209 or early 1210. The failure of this attack prompted Michael to break away from his Bulgarian allies and support the Latins. In early 1211, Strez clashed with the Latins and Epirotes at Thessaloniki and required Boril's assistance after Michael and Henry invaded the western reaches of Strez's realm. In the early summer, the allied Bulgarian army suffered a heavy defeat at Bitola at the hands of Michael, Henry's brother Eustace and Bernard of Katzenellenbogen. Even though it resulted in no territorial losses, it prevented