Abu Ishak Muhammad ibn Harun al-Rashid, given the regnal title al-Mu'tasim Bi'llah, was the eighth Abbasid caliph and founder of the imperial capital city of Samarra. Before ascending to the throne, Mu'tasim had been a military commander, and some historical sources suggest that he founded Samarra in part to accommodate the growing army that he had amassed as a form of protection.
Sources:
C. E. Bosworth. "al-Mu'taṣim Bi ’llāh." Encyclopaedia of Islam, Second Edition. http://dx.doi.org/10.1163/1573-3912_islam_SIM_5656 [Accessed November 21, 2017].