Al-Mustaʿṣim bi-llāh (; 1213 – 20 February 1258) was the 37th and last caliph from the Abbasid dynasty ruling from Baghdad. He held the title from 1242 until his death in 1258. Biography Abu Ahmad Abdallah ibn al-Mustansir Billah () was a son of the Abbasid caliph al-Mustansir, and his mother was Hajir. He was born in 1213. After the death of his father, al-Musta'sim succeeded to the throne in late 1242. He is noted for his opposition to the rise of Shajar al-Durr to the Egyptian throne during the Seventh Crusade. He sent a message from Baghdad to the Mamluks in Egypt that said: "If you do not have men there tell us so we can send you men." However, al-Musta'sim had to face the greatest menace against the caliphate since its establishment in 632: the invasion of the Mongol forces that, under Hulagu Khan, had already wiped out any resistance in Transoxiana and Khorasan. In 1255/1256 Hulagu forced the Abbasid to lend their forces for the campaign against Alamut. News of the murder of al-Muazzam Turanshah and the inauguration of Shajar al-Durr as the new Sultan reached Syria. The Syrian Emirs were asked to pay homage to Shajar al-Durr but they refused and the Sultan's deputy in Al Karak rebelled against Cairo. The Syrian Emirs in Damascus gave the city to an-Nasir Yusuf the Ayyubid Emir of Aleppo and the Mamluks in Cairo responded by arresting the Emirs who were loyal to the Ayyubids in Egypt. In addition to the Ayyubids in Syria, the Abbasid caliph al-Musta'sim in Baghdad also rejected the Mamluk move in Egypt and refused to recognize Shajar al-Dur as a Sultan. The refusal of the Caliph to recognize Shajar al-Durr as the new Sultan was a great setback to the Mamluks in Egypt as the custom during the Ayyubid era was that the Sultan could gain legitimacy only through the recognition of the Abbasid Caliph. The Mamluks, therefore, decided to install Izz al-Din Aybak as a new Sultan. He married Shajar al-Durr who abdicated and passed the throne to him after she had ruled Egypt as sultan for about three months. Though the period of Shajar al-Durr's rule as a sultan was of short duration, it witnessed two important events in history: one, the expelling of Louis IX from Egypt, which marked the end of the Crusaders' ambition to conquer the southern Mediterranean basin;