Final answer:
Askia the Great joined the military to expand and control the Songhai Empire's territory and trans-Saharan trade routes, and to establish authority through Islam, which was strategic for trade and military expansions.
Step-by-step explanation:
Askia M. Touré, also known as Askia the Great, joined the military and later became a ruler largely to extend the territory of the Songhai Empire and to control the trans-Saharan trade routes. His military conquests, such as capturing the salt-producing center of Taghaza, directly contributed to the economic strength of the empire. Askia the Great centralized his power through the military while elevating the status of the Songhai Empire to be the largest in West Africa's history.
Furthermore, Islam was instrumental in legitimizing Askia the Great's rule, as he was not ethnically Songhai. By declaring a holy war against the non-Muslim Mossi and making a pilgrimage to Mecca, he not only fortified his religious authority but also his political one as he secured recognition from the caliph in Egypt. These actions were intertwined with military expansions and trade regulation, showcasing the multidimensional role of the military under his leadership.