After the death of Ivan Assen the most influential boyar families started bloody internecine struggles for the throne, which were skilfully used by Byzantium for enhancing its influence among the Bulgarian aristocracy. During the reign of Konstantin Assen( 1257-1277), who lacked will power and was incapable as a ruler, a number of feudal lords separated themselves from the central power, while the Byzantines conquered vast Bulgarian territories in Thrace and Macedonia. Konstantin Assen married the Byzantine princess Maria, after which the Byzantines began openly to command in Turnovo.
The unceasing feudal internecine struggles and the frequent incursions by Byzantine troops resulted in economic ruin and sharp deterioration of the situation of the peasant masses, which became disastrous when the Tartars made their appearance in the North – a numerous and belligerent people from the steppes who had already subordinated most of the divided Russian principalities. The boyars, hidden behind the stone walls of their fortresses, left the defenceless peasants at the mercy of the Tartars. Konstantin Assen himself became a vassal of the Tartar Khan Nogai,
The people, left to themselves, had to think about their own defense. In this struggle, the name of the swineherd Ivailo from Northeastern Bulgaria gained ever increasing popularity. In 1277 at the head of a group of dare-devils, he routed several Tartar detachments which had set out to loot the villages. Ivailo’s glory spread with lightning speed throughout the country and soon he had gathered a whole army under his colours, consisting mainly of desperate peasants in search of protection. The Tartars were chased to the other bank of the Danube and then came the turn of the avid and venal boyars, who had caused to the people even more sufferings than the Tartars.
In the style of mediaeval superstition, Ivailo declared that he had heard a voice from heaven ordering him to save his people from those who looted and tortured them. The peasant volunteers in his army, who had risen to de-fend their land and homes from the foreign invaders, turned into an insurgent army which captured a number of feudal castles, storehouses for food and arms and was headed for the capital. The tsar, who had not dared come out of Turnovo’s walls while Ivailo’s peasant forces were shedding their blood to repel the Tartars, rallied his army and set out against Ivailo’s ‘rabble’. The latter, steeled in the cruel battles with the Tartar hordes, defeated the army of the Tsar in the very first encounter. The Tsar himself fell in the battle.