The group was created as successor to Muqtada al - Sadr 's Mahdi Army he also called on other Special Groups to join the brigade .
Muqtada al - Sadr 's Mahdi Army was Iraq 's largest Shi'a militia until its disbanding in 2008 .
The group was created as successor to Muqtada al - Sadr 's Mahdi Army , which was Iraq 's largest Shi'a militia until its disbanding in 2008 , he also called on other Special Groups to join the brigade .