Valentinian I was elevated to the position of Roman emperor due to his proven military abilities and the unwavering support he garnered from his troops. Upon the death of Emperor Jovian, the Roman Empire found itself in a precarious position with threats looming both from outside its borders and within its political structure. The troops, in a demonstration of their loyalty, proclaimed Valentinian as emperor in Nicaea in February 364. His military background, having served as a general, positioned him as a fitting candidate to address the pressing challenges of the time.

Upon assuming power, Valentinian recognized the need for effective governance and stability amidst the turbulence facing the Empire. He took immediate steps to secure the empire’s borders and protect it from the advancing barbarian tribes. His strategic decisions led to important military campaigns aimed at fortifying and expanding the Empire's territories. Furthermore, he appointed his brother, Valens, as co-emperor, which signified the beginning of a dual leadership structure that would play a pivotal role in the governance of the increasingly divided Roman Empire.

Valentinian's rule is marked by notable actions, including reforms within the military, and attempts to strengthen the imperial authority that had been challenged. His reign, however, was not devoid of strife, as he faced civil discord as well as religious tensions, which were becoming increasingly prominent during this period in the Roman Empire. Valentinian's decisions and policies would strongly influence the course of Roman history as his reign set the stage for the eventual division of the Empire and the struggles that would follow.