With each legion having 5,120 legionaries usually supported by an equal number of auxiliary troops (according to Tacitus), the total force available to a legion commander during the Pax Romana probably ranged from 11,000 downwards, with the more prestigious legions and those stationed on hostile borders or in restive provinces tending to have more auxiliaries. By the time of the emperor Severus, 193-211, the auxiliaries may have composed 55 to 60% of the army, 250,000 of 447,000. Some legions may have even been reinforced at times with units making the associated force near 15,000–16,000 or about the size of a modern division. Throughout the imperial era, the legions played an important political role. Their actions could secure the empire for a usurper or take it away. For example, the defeat of Vitellius in the Year of the Four Emperors was decided when the Danubian legions chose to support Vespasian. In the empire, the legion was standardized, with symbols and an individual history where men were proud to serve. The legion was commanded by a "legatus" or "legate". Aged around thirty, he would usually be a senator on a three-year appointment. Immediately subordinate to the legate would be six elected "military tribunes" – five would be staff officers and the remaining one would be a noble heading for the Senate (originally this tribune commanded the legion). There would also be a group of officers for the medical staff, the engineers, record-keepers, the "praefectus castrorum" (commander of the camp) and other specialists such as priests and musicians. In the Later Roman Empire, the number of legions was increased and the Roman Army expanded. There is no evidence to suggest that legions changed in form before the Tetrarchy, although there is evidence that they were smaller than the paper strengths usually quoted. The final form of the legion originated with the elite "legiones palatinae" created by Diocletian and the Tetrarchs. These were infantry units of around 1,000 men rather than the 5,000, including cavalry, of the old Legions. The earliest "legiones palatinae" were the "Lanciarii", "Joviani", "Herculiani" and "Divitenses". The 4th century saw a very large number of new, small legions created, a process which began under Constantine II. In addition to the elite "palatini", other legions called "comitatenses" and "pseudocomitatenses", along with the "auxilia palatina", provided the infantry of late Roman armies.