The Roman Empire was radically transformed during the fifth century, which effectively extends from 395 to 518. Theodosius the Great had recognized that simultaneous pressure from Persian kings in the East and migrating German tribes in the West required a continual imperial presence on both fronts. That and a desire for imperial unity necessitated sharing the imperial office between two or more colleagues dynastically linked by marriage, blood, or both. Various problems and weaknesses contributed to the undermining of Roman power more in the West than in the East. The West faced a greater number of external threats along more permeable frontiers and could no longer depend on surplus revenues from the East. The Germanic tribes had invaded the Roman Empire partly because of pressure from the Huns. Pastoral nomads, the Huns may have originated on the steppes of what is Kazakhstan.