Constantine I, often called Constantine the Great, was one of the most significant emperors of ancient Rome, reigning from 306 to 337 AD. Born in Naissus, in the province of Dacia Mediterranea (modern-day Niš, Serbia), he was the son of Flavius Constantius, a Roman army officer and a prominent figure in the Tetrarchy, and Helena, a woman of humble Greek origins. His father's position as one of the four emperors in the Tetrarchy was instrumental in shaping Constantine's early life and military career. Under emperors Diocletian and Galerius, Constantine demonstrated his military prowess, initially fighting against barbarian tribes and the Persians in the eastern provinces. In 305 AD, he was summoned to the West to aid his father in campaigns in Britain, where he earned further distinction.

Following the death of his father in 306 AD, Constantine's soldiers declared him emperor at Eboracum (modern York, England). Over the next two decades, he would overcome rival emperors, such as Maxentius and Licinius, in a series of civil wars, eventually becoming the sole ruler of the Roman Empire by 324 AD. His reign marked a period of significant transformation, both militarily and administratively.

To strengthen the empire, Constantine initiated a series of reforms. He reorganized the imperial administration, separating civil and military powers to create a more efficient system of governance. Acknowledging the need for economic stability, he introduced the solidus, a new gold coin that helped curb inflation. This currency would become the standard for Byzantine and European economies for nearly a millennium. He also restructured the Roman army, creating specialized units that could respond swiftly to internal threats and barbarian invasions.