 Complete the following phrase:  During the 12th and 13th centuries, Bulgarians and Serbs took advantage of Byzantine decline to create large kingdoms stretching from Danube to the Aegean Sea. Kaloyan brought Skopje back into reestablished Bulgaria in 1203 until his nephew Strez declared autonomy along the Upper Vardar with Serbian help only five years later. In 1209 Strez switched allegiances and recognized Boril of Bulgaria with whom he led a successful joint campaign against Serbias first internationally recognized king Stefan Nemanjić. From 1214 to 1230 Skopje was a part of Byzantine successor state Epirus before recaptured by Ivan Asen II and held by Bulgaria until 1246 when the Upper Vardar valley was incorporated once more into a Byzantine state  the Empire of Nicaea. Byzantine conquest was briefly reversed in 1255 by the regents of the young Michael Asen I of Bulgaria. Meanwhile, in the parallel civil war for the Crown in Tarnovo Skopje boyar and grandson to Stefan Nemanja Constantine Tikh gained the upper hand and ruled until Europes only successful peasant revolt the 