Under the late republic, the number of provinces rapidly increased, and therefore, Pompey the Great proposed a new law, the Lex Pompeia de provinciis , in which former praetors and consuls were obliged to become governor five years after their term in office (53). At more or less the same time, he had himself elected as governor of several provinces, which were not governed by himself, but by his representatives, the legati . The emperor Augustus copied this idea when he changed the empire, until then ruled as a republic, into a monarchy. He was made governor of almost all provinces with legions, and used legati to rule them. At the same time, the rest of the empire was governed by proconsuls. So, there were two types of governors: Proconsuls. In fact, these men were not former consuls, but former praetors. They governed the senatorial provinces and typically served twelve months. Only the rich provinces -Asia and Africa- were entitled to a proconsul who was indeed an ex-consul. Legati Augusti pro praetore. These men served in the emperors provinces with the armies (the imperial provinces ). Usually, their term in office lasted thirty-six months, although the emperor Tiberius preferred longer terms. There was a third group of governors. In several unimportant provinces, prefects were appointed. Usually, these military men governed parts of larger provinces. The best known example is Pontius Pilate, who governed Judaea, an annex to Syria. Prefects were not senators but knights. Egypt was also governed by a prefect, not because it was unimportant, but because it was the emperors own possession. When Septimius Severus conquered Mesopotamia, he used the same construction. After the mid-first century, the prefects were gradually replaced by procurators (except for Egypt). The only difference is that prefects were soldiers and procurators were fiscal officials. It tells something about the success of the Pax Romana .