 Complete the following phrase:  Initially, the Ottoman invasion was not considered as a significant threat by both Bulgarians and Byzantines. For only one decade between 1354 and 1364 the Ottomans conquered virtually the whole of Thrace seizing large cities such as Plovdiv, Beroia, Dianopolis Yambol and Adrianople and defeating several small Bulgarian forces. The centuries-old mistrust between Bulgarians and Byzantines spoiled the negotiations between the two empires for an alliance and even led to the last Byzantine-Bulgarian war in 1364. In 1371 a large Bulgarian-Serb army under Vukašin Mrnjavčević and Jovan Uglješa, two feudal lords in Macedonia, was annihilated by the Ottomans under Lala Shahin Pasha at Chernomen and soon the Bulgarian Emperor had to admit the defeat and became a vassal to the invaders. Numerous Bulgarian fortresses in the Rhodope mountains, Sofia valley and eastern Bulgaria were captured one by one over the next twenty years. In 1393 the capital Tarnovo was 