The Byzantine Emperor Michael VIII Palaiologos tried to exploit this situation and intervened in Bulgaria. He sent Ivan Asen III, son of the former Emperor Mitso Asen, to claim the Bulgarian throne at the head of a large Byzantine army. Simultaneously, Michael VIII incited the Mongols to attack from the north, forcing Ivaylo to fight on two fronts. Ivaylo was defeated by the Mongols and besieged in the important fortress of Drastar. In his absence, the nobility in Tarnovo opened the gates to Ivan Asen III. However, Ivaylo broke the siege and Ivan Asen III fled back to the Byzantine Empire. Michael VIII sent two large armies, but they were both defeated by the Bulgarian rebels in the Balkan mountains.

Meanwhile, the nobility in the capital had proclaimed as emperor one of their own, the magnate George Terter I. Surrounded by enemies and with diminishing support due to the constant warfare, Ivaylo fled to the court of the Mongol warlord Nogai Khan to seek aid, but was eventually murdered. The legacy of the rebellion endured both in Bulgaria and in Byzantium.