Selim I's conquest of Egypt in 1517 gave the Ottomans the guardianship of Mecca and Medina, the title the sultans actually used of themselves, servant of the two holy sanctuaries, and possession of the Cairo Abbasid line. The story that the last Cairo Abbasid formally transferred the caliphate to Selim is a later construction, and historians treat it with caution; what is not in doubt is that the sultans exercised caliphal functions and that the claim became politically central in the nineteenth century, when Abdulhamid II used it to appeal to Muslims under British, French and Russian rule.
The nineteenth century reforms cut the other way. The edicts of 1839 and 1856 promised equality before the law to all subjects regardless of religion, and the 1856 edict was obtained under allied pressure at the end of the Crimean War; a nationality law and mixed courts followed. The result satisfied nobody: Muslim opinion resented the loss of precedence, and Christian communities increasingly wanted national states rather than equal subjecthood.
The end came quickly. The empire entered the First World War on the German side and proclaimed a jihad in 1914 that Muslims elsewhere largely ignored, and defeat brought occupation and a nationalist movement in Anatolia. The Grand National Assembly abolished the sultanate in 1922 and, on 3 March 1924, the caliphate itself, an act whose shock across the Muslim world Mona Hassan traces and which produced a congress at Cairo in 1926 that failed to replace it.