Abd Allah al-Mahdi Billah

al-Mahdi Billah المهدي بالله
Gold dinar of al-Mahdi bi'llah, minted in Kairouan, 912 CE
Imam–Caliph of the Fatimid Caliphate
Reign	27 August 909 – 4 March 934
Successor	al-Qa'im bi-Amr Allah
Born	Saʿīd ibn al-Ḥusayn 31 July 874 Askar Mukram, Khuzistan
Died	4 March 934 (aged 59-60) Mahdiya
Issue	al-Qa'im bi-Amr Allah
Regnal name Abu Muhammad Abdallah al-Imam al-Mahdi Billah
Dynasty	Fatimid
Father	al-Husayn ibn Ahmad (Abdallah al-Radi)
Religion	Isma'ili Shi'a Islam

Abū Muḥammad ʿAbd Allāh ibn al-Ḥusayn (Arabic: أبو محمد عبد الله بن الحسين; 31 July 874 – 4 March 934), better known by his regnal name al-Mahdī biʾllāh (Arabic: المهدي بالله, "The Rightly Guided by God"), was the founder of the Isma'ili Fatimid Caliphate, the only major Shi'a caliphate in Islamic history, and the eleventh Imam of the Isma'ili branch of Shi'ism.

He was born as Saʿīd ibn al-Ḥusayn (سعيد بن الحسين) in Askar Mukram to a family that led the secret Isma'ili missionary network (da'wa), propagating on behalf of the hidden imam, Muhammad ibn Isma'il, who would return as the prophesied Islamic messiah (mahdi). Orphaned at a young age, he moved to Salamiya, the family's base of operations, where he was adopted by his uncle. In the mid-890s Sa'id succeeded to the leadership of the expanding da'wa, which had expanded and gained adherents across the then Muslim world. However, his claims of not merely being a trustee of the hidden imam, but of him and his ancestors holding the imamate itself, led in 899 to a schism in the Isma'ili movement: those who did not recognize his claims split off to become the Qarmatians. The schism was followed by uprisings of pro-Isma'ili Bedouin in Syria in 902–903, launched without his consent by over-eager supporters, who aimed to force him to come forward as the mahdi. The Bedouin uprising was suppressed by the Abbasids, but drew the attention of the Abbasid Caliphate's authorities to him, forcing him to abandon Salamiya, and flee first to Ramla, then Fustat in Egypt, and finally Sijilmasa in what is now Morocco. There he remained, living as a merchant, until one of his missionaries, Abu Abdallah al-Shi'i, at the head of the Kutama Berbers overthrew the Aghlabid dynasty of Ifriqiya in 909.

Proclaimed caliph and assuming power in Ifriqiya in January 910, he proclaimed his right to conquer the world in the name of God, but soon fell out with Abu Abdallah and other leading missionaries, who were disappointed that he was not the semi-divine mahdi they had been propagating for. Al-Mahdi was able to purge these dissidents, but had to overcome a series of revolts against his authority, either due to opposition to the exactions of the Kutama, the backbone of his power, or due to disillusionment of his followers with his failure to realise the Isma'ili millennialist promises. The state that al-Mahdi built, although underpinned by a messianic ideology, was otherwise conventionally organized, and relied heavily on the personnel of the previous Aghlabid regime and the swords of the Kutama. His expansionist aims achieved only moderate success: two invasions of Egypt were beaten back by the Abbasids, leaving only the Cyrenaica in his hands, while the war with the Byzantine Empire in southern Italy was characterized by raids for plunder and slaves, and did not result in any lasting successes. In the west, his repeated attempts to impose Fatimid rule over the unruly Berbers were challenged not only by Berber rivalries, but also by the Umayyads of al-Andalus, and only secured temporary success. In 921 he moved his court to the newly built fortified palace city of Mahdiya on the Tunisian coast, and spent the rest of his life there. After his death in 934, he was succeeded by his only son, al-Qa'im.