Strez (Bulgarian and ; original spelling: Стрѣзъ) (fl. 1207–1214) was a medieval, semi-independent Bulgarian sebastokrator. He was a member of the Asen dynasty and a cousin or a brother of Boril of Bulgaria. A major contender for the Bulgarian throne, Strez initially opposed the ascension of his close relative Tsar Boril. He fled to Serbia, where he accepted the vassalage of Grand Prince Stefan Nemanjić, and Serbian support helped him establish himself as a largely independent ruler in a large part of the region of Macedonia. However, Strez turned against his suzerains to become a Bulgarian vassal and joined forces with his former enemy Boril against the Latins and then Serbia. Strez died amidst a major anti-Serbian campaign under unclear circumstances, sometimes described as a Serbian plot.
Throne contender and Serbian vassal
Nothing is mentioned of Strez until the events in the wake of the sudden death of Tsar Kaloyan (1197–1207) during his siege of the Crusader-held Thessaloniki. Just like Alexius Slav, another noble who would later emerge as a separatist, Strez was a nephew of the Asen brothers Peter, Ivan Asen and Kaloyan, who were the first three emperors of the Second Bulgarian Empire. However, it is unclear whether through his relation to the early Asens he was a first cousin or a brother of Boril (1207–1218).
At the time of Kaloyan's death, Strez was in the capital Tarnovo, perhaps seeking to capitalize on his ancestral rights to the Bulgarian crown. However, Boril proved to be the more ambitious candidate. Boril persecuted the other candidates for the throne, and Alexius Slav, along with Ivan Asen's sons Ivan Asen II had to leave Bulgaria.
As happened to other members of the royal family, Boril's ascension forced Strez and his closest supporters to flee, in that case to neighbouring Serbia, where he was welcomed by the reigning Stefan Nemanjić (1196–1228) in 1207 or early 1208. Even though Boril requested the extradition of Strez to Bulgaria, the Serbian ruler hoped to use Strez as a puppet in gaining Bulgarian-held territory. Stefan believed that Strez's royal ancestry and imperial aspirations would make it much easier to impose Serbian rule over Macedonia, Kosovo and Braničevo, as well as Belgrade, all captured by Bulgaria under Kaloyan. At the same time, Boril was unable to take military action against Strez and his Serbian patron, as he had suffered a major defeat at the hands of the 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 Vlach 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.