Al-Fadl ibn al-Rabi (Arabic: الفضل بن الربيع, 757/8–823/4), was one of the most influential officials of the Abbasid Caliphate in the reigns of Harun al-Rashid (r. 786–809) and al-Amin (r. 809–813), whom he served as chamberlain and chief minister.
[1][5] However, Fadl lacked the almost plenipotentiary powers that Harun had granted Yahya, and his remit was limited to a supervisory role over expenditure and in the handling of petitions, correspondence and execution of orders to the Caliph, while the actual financial administration was entrusted to another official.
[1][6] Amin, who had need of Fadl's experience, sent letters to him urging him to return to the capital, and to bring with him the treasury, which Harun had taken along, as well as the entire expeditionary army assembled to crush the rebellion.
After Ma'mun's forces scored an unexpected victory over the caliphal army at the Battle of Ray, the situation became critical in Baghdad, where many began to accuse Amin of idleness and complacency and Fadl of inefficient leadership.
[11] As Ma'mun's general Tahir ibn Husayn advanced through Iran, Fadl tried to reinforce the Baghdad troops (the abna al-dawla) with levies from the Arab tribes of Syria and the Jazira, but they soon fell out with the abna′, who were jealous of their pay and privileges, so that this project came to nothing.
[8][9] Despite his long and loyal service to the Abbasids, Fadl's assessment by modern historians is negative, as he is considered the main instigator of the civil war through his machinations to remove Ma'mun from the succession.
[1][15] Thus Dominique Sourdel calls him "an intriguer of mediocre personality and limited ability" who tried to use Amin's weak character for his own advantage,[9] while Kennedy sees in him the "evil genius" responsible for the destructive civil war.