The Imperial Army established by Augustus drew heavily on the nomenclature and traditions of the late Roman Republic, but was revolutionary in its design. He decided to meet all the military needs of the Empire from a standing, professional army. Military service became a career: enlistment was for 25 years (16 in the Praetorian Guard), and men were sometimes retained even longer. The loyalty of the new army was to the emperor and not to either the Senate or the People of Rome. Imperial legions became permanent units with their own numbers and titles and many were to remain in existence for centuries to come.
Read an extract of The Roman Army of the Principate 27 BC–AD 117