Valentinian I was a Roman Emperor who ruled from 364 to 375 AD. He is known for his military campaigns against various tribes, including the Goths and Sarmatians, which helped to secure the borders of the Roman Empire. Valentinian I established a strong administrative system and appointed capable generals to maintain stability. He divided the empire into eastern and western regions, appointing his brother Valens to govern the East. Valentinian I's reign was marked by efforts to strengthen the military and improve the economy, but it also faced challenges, including internal strife and external threats from invading groups.