.. . al -Mahdi (Arabic: أبو عبد الله محمد بن عبد الله المنصور بن محمد بن علي المهدي بالله, DMG Abū ʿ Abd Allāh Muḥammad b ʿ Abd Allāh al - Mansur b Muḥammad b ʿ Alī al -Mahdi bi- ' llāh; * 743 / 44 or 745; † August 4 785) was the third Abbasid Caliph ( 775-785 ).

As designated successor took over al -Mahdi 775 the reign of his father, al - Mansur. Even under the reign of his father, he had as a commander in the conquest of Tabaristan south of the Caspian Sea Award ( 756 ), since 758/59, he served also as governor of Rayy.

Under his reign the expansion of imperial administration was continued and maintained the stance al - Mansur. Also al -Mahdi As this was based primarily on Abbasid family members. Unlike his two predecessors, al -Mahdi but seems to have the religious aspect of his duties given more weight, which is also apparent from the elected rulers name " al -Mahdi ".

Apart from the uprising of Muqanna in Khorasan ( 775/76-779 ) and the ongoing civil unrest of the Kharijites in Ifriqiya / Maghreb and Iran, the kingdom was at peace, his reign is characterized as a peace and prosperity time. Under Prince Harun ( al-Rashid ) began 780 in response to hostile attacks again campaigns against Byzantium. 782 Muslim troops advanced further in front on Byzantine territory and were able to enforce Byzantine Tribute. Compared to the Alid al -Mahdi tried to pursue a policy of reconciliation; at least he reached that in this respect there were no major incidents during his reign.

In contrast to the " modest " Hofhaltung al - Mansur al -Mahdi operation after Persian models (mainly based on the Sassanids ) a very magnificent court life in Baghdad and promoted culture and science. The learned Syrian Christian Theophilus of Edessa worked at his court as an astrologer and was sponsored by al - Mahdi.

Towards the end of the government led to the harem intrigue, as the Berber harem girl Haizuran gained influence and wanted her son Harun secure the throne. However, al -Mahdi confirmed the succession of his son Musa (al- Hadi ) ( 785-786 ), however, put firmly Harun al-Rashid as his successor.