Roman expansion began in the days of the Republic, but reached its zenith under Emperor Trajan. At this territorial peak, the Roman Empire controlled approximately 6,5 million km²[6] of land surface. Because of the Empire's vast slotmachine extent and long endurance, Roman influence upon the language, religion, architecture, philosophy, law, and government of nations around the world lasts to this day.

In the late 3rd century AD, Diocletian established the practice of dividing authority between four co-emperors, in order to better secure the vast territory. During the following decades the empire was often divided along an East/West axis. After the death of Theodosius in 395 it was divided for the last time.