Theodosius I (Ancient Greek: Θεοδόσιος Theodosios; 11 January 347 – 17 January 395), also known as Theodosius the Great, was Roman emperor from 379 to 395. He won two civil wars and was instrumental in establishing the Nicene Creed as the orthodox doctrine for Nicene Christianity. Theodosius was the last emperor to rule the entire Roman Empire before its administration was permanently split between the Western Roman Empire and the Eastern Roman Empire. He ended the Gothic War (376–382), but did so on terms disadvantageous to the empire, with the Goths remaining politically autonomous within Roman territory, albeit as nominal allies.
Born in Hispania, Theodosius was the son of a high-ranking general of the same name, Theodosius the Elder, under whose guidance he rose through the ranks of the Roman army. Theodosius held independent command in Moesia in 374, where he had some success against the invading Sarmatians. Not long afterwards, he was forced into retirement, and his father was executed under obscure circumstances. Theodosius soon regained his position following a series of intrigues and executions at Emperor Gratian's court.