Complex sentence: The revolt was fuelled mainly by the failure of the central authorities to confront the Mongol menace over north-eastern Bulgaria, who had looted and ravaged the Bulgarian population for decades, especially in the region of Dobrudzha.
Split into simpler sentences: The revolt was fuelled mainly by the failure of the central authorities to confront the Mongol menace over north-eastern Bulgaria. The Mongols had looted and ravaged the Bulgarian population for decades, especially in the region of Dobrudzha.