Al-Mu'tasim Billah was the eighth caliph of the Abbasid Caliphate, ruling from 833 to 842 CE. He was known for his military campaigns and efforts to strengthen the caliphate's authority. His reign marked a period of significant military expansion, particularly against the Byzantine Empire. During his rule, Al-Mu'tasim Billah established the city of Samarra as the new capital, which became a center of culture and administration. He also focused on building a strong army, including the use of Turkic soldiers, which played a crucial role in his military strategies and governance.