Islam is founded on the Constitution of Medina which was drafted by Muhammad (c. 570-632) in the 7th century, essentially creating a constitutional theocracy. Following the death of Muhammad a successor, the caliph, was appointed to serve as head of the Islamic State which became the Caliphate. A dynastic chain of succession unfolded with the following series of Caliphates: Rashidun Caliphate (632–661), Umayyad Caliphate (661–750), Abbasid Caliphate (750–1258) and the Ottoman Caliphate (1517–1924).
The Ottoman Caliphate was abolished by the Grand National Assembly of the Republic of Turkey on March 3, 1924, the last Caliph Abdülmecid II and his family were banished from Turkey and lived out their days in exile. Efforts to restore the Caliphate and the authority of Islam in temporal affairs got underway very soon after. The Muslim Brotherhood was founded in Egypt in 1928 by Hassan al-Banna whose members adhere to the credo "Allah is our objective; the Quran is our law, the Prophet is our leader; Jihad is our way; and death for the sake of Allah is the highest of our aspirations."
Unfortunately, nostalgia for the restoration of the Caliphate and the inability of much of the Islamic world to develop separation of religion and state (as in the West) has given rise to totalitarianism (as is the case in Iran) and religious intolerance, as seen in internecine fighting between the two most prominent Islamic denominations: Sunni and Shia.
It took several centuries and a great deal of bloodshed for the Western world to develop the separation of religion and state while preserving religious liberty. The last thing we need is a return to the sectarian strife of a sort Western society endured in the Middle Ages.