From 937, Abu Yazid began to openly preach holy war against the Fatimids. Abu Yazid conquered Kairouan for a time, but was eventually driven back and defeated by the Fatimid caliph al-Mansur bi-Nasr Allah. Abu Yazid's defeat was a watershed moment for the Fatimid dynasty. As the historian Michael Brett comments, "in life, Abu Yazid had brought the Fatimid dynasty to the brink of destruction; in death he was a godsend", as it allowed the dynasty to relaunch itself following the failures of al-Qa'im's reign.