Final answer:
The decline of the Abbasid Caliphate was due to cultural conflicts with religious conservatives, a weakened political bureaucracy and military reliance on mamluks, and the rise of powerful rivals leading to territorial loss and fragmentation.
Step-by-step explanation:
The decline of the Abbasids is attributed to several key factors. Firstly, the worldliness and cosmopolitanism of the Abbasid culture led to unrest among religious conservatives like the scholar al-Ghazali, who criticized the incorporation of non-Islamic elements such as Greek speculative thought into Islamic beliefs. Secondly, the political structure of the caliphate weakened due to an entrenched and often perceived as corrupt bureaucracy, the reliance on mamluks, and the shift of effective power to regional governors and local authorities, leading to political devolution. Lastly, the emergence of powerful rivals such as the Fatimid Caliphate and the Seljuk Turks, alongside the establishment of independent dynasties, greatly diminished the unifying control of the Abbasids.
These developments culminated in the fragmentation of the Islamic world and the eventual seizing of Abbasid territories by competitors, leaving the once mighty Abbasid empire vulnerable to external threats, such as the European Crusaders and, ultimately, the Mongol invasion that ended their rule in 1258