With the threat from Ivaylo and Ivan Asen III removed, George Terter I made an alliance with King Charles I of Sicily, with Stefan Dragutin of Serbia, and with Thessaly against Michael VIII Palaeologus of the Byzantine Empire in 1281.

The alliance failed as Charles was distracted by the Sicilian Vespers and the secession of Sicily in 1282, while Bulgaria was ravaged by the Mongols of the Golden Horde under Nogai Khan.

Theodore Svetoslav also returned to Bulgaria after a successful mission of Patriarch Joachim III and was made co-emperor by his father, but after another Mongol invasion in 1285, he was sent off as a hostage to Nogai Khan.

[1] The Byzantine Emperor Andronikos II Palaiologos at first refused to receive him, perhaps fearing complications with the Mongols, and George Terter was kept waiting in wretched conditions in the vicinity of Adrianople.

In 1301 his son Theodore Svetoslav, already emperor of Bulgaria, defeated a Byzantine army and captured thirteen high-ranking officers, whom he exchanged for his father.