However, Strez turned against his suzerains to become a Bulgarian vassal and joined forces with his former enemy Boril against the Latins and then the Serbs.
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Some ruins by the Vardar River near Jegunovce west of Skopje were known to the locals as "Strez's Fortress" (Стрезово кале, Strezovo kale).
Strez was for a time a Duke under Stefan Nemanjić and had by 1209 conquered most of Macedonia; from the Struma valley in the east, which bordered lands controlled by Boril, to Bitola and perhaps Ohrid in the west, and from Skopje in the north to Veria in the south.