Fatimid expansion peaked when a Baghdad general proclaimed their caliph for a year

Evidence

The high water mark of Fatimid expansion came in the years 448–451/1057– 9, when a Turkish general in Iraq called Arslan al-Basasiri went over to the Fatimid side and proclaimed the Fatimid Caliph first in Mosul and then, for a year, in Baghdad itself. Despite the efforts of the Chief Missionary, however, the Fatimid government was unable to provide effective support, and the strongly Sunni Seljuqs drove al-Basasiri out of Baghdad. The Ghaznavid ruler in the East had already opted for Sunnism, to which he brought powerful reinforcement. The Isma ilis of Multan were crushed—those of Persia and Iraq subjected to both repression and counter-propaganda. — Bernard Lewis, From Babel to Dragomans, 412