Abu Ja'far Abdallah ibn Muhammad al-Mansur usually known simply as by his laqab Al-Mansur was the second Abbasid caliph, reigning from 754 CE – 775 CE and succeeding As-Saffah. He is known for founding the 'Round City' of Madinat al-Salam, which was to become the core of imperial Baghdad. Modern historians regard Al-Mansur as the real founder of the Abbasid Caliphate, one of the largest polities in world history, for his role in stabilizing and institutionalizing the dynasty.