Death of Caliph al-Mustansir Billah and accession of al-Musta'sim in 640

The caliph Abu Jaʿfar al-Mustansir bi-Llah Mansur ibn al-Zahir died on the morning of Friday, the tenth of Jumada II 640 AH, and the people mourned him greatly. The oath of allegiance was given to his son ʿAbd Allah al-Musta'sim bi-Llah ¹.

He was fair-haired, large and short in stature. Grey hair had appeared on him and he dyed it with henna, then left it. He was generous, kind, gentle and just. He built the Mustansiriyya college and endowed it for the four legal schools, and a hospital and a bath house. There was nothing like it in the world, and in Iraq it was like the Great Mosque of Damascus. He built mosques, hospices and inns along the roads, and money had no measure for him. His father al-Nasir had built a reservoir and left money in it, saying, "I wonder whether I will live to fill it." When al-Mustansir came to power he would say, "I wonder whether I will live to empty it" ^{1 3}.

With his death closed seventeen years that were the last period of the Baghdad caliphate's expansion and prosperity, before al-Musta'sim inherited a state that had to face the Hulagids.