Selim I "the Grim," over the course of his brief reign 1512-1520, secured Mecca, Medina, and Jerusalem, the three Islamic holy cities, and utterly demolished the Mamelukes of Egypt, who had been seen as the holders/protectors of the Holy Cities. Selim's conquests totally changed the character of the Ottoman holdings, which had previously been majority Christian and heavily European, into a truly Eastern Mediterranean empire with large Muslim populations in Syria and Egypt added. With the collapse of the Mamelukes, the possession of the holy cities, and the rivalry with the Shi'ite Safavids, proclaiming the Ottoman sultan the successor to the caliph tradition and the commander of the faithful etc. was just the natural next step.

In short, Selim became the first Ottoman caliph in 1517 after his dramatic conquest of all the Mameluke holdings (Egypt, the Levant, and the Hedjaz).