Answer:
Internal termoil and foreign conquest
Explanation:
It started to lose strength (partially due to ethic tensions) since it was formed,and the Seljuks from the east and Serbians from the north took anatolian territories(Seljuks,later to become Ottomans) and today's Greek territories(annexed and puppeted by the Serbian empire (who took the position of the orthodox powerhouse from the Byzantines) over the 14th century.Later it fell to the Ottomans
