This work explores the reign of Theodosius I, the last emperor to rule over a unified Roman Empire, analyzing his military, political, and religious reforms as well as their lasting impact on the empire's trajectory. Theodosius' reign (379–395 AD) marked a pivotal turning point in Roman history, as he navigated internal divisions, external invasions, and the transformation of the empire into a Christian state. His efforts to stabilize the empire, including military reforms, the integration of barbarian groups (such as the Goths) into the Roman military as foederati, and his enforcement of Nicene Christianity as the state religion, played a crucial role in shaping the late Roman world. However, his policies also exposed the empire's vulnerabilities, particularly in the Western half, where the growing dependence on barbarian federates and internal fragmentation led to the eventual collapse of the Western Roman Empire. The division of the empire between his sons, Arcadius and Honorius, further accelerated the decline of the West while the East endured, ultimately evolving into the Byzantine Empire. Through a detailed examination of Theodosius' legacy, this study sheds light on his critical role in the transition from the ancient Roman world to the medieval order, highlighting both the achievements and limitations of his reign.