During and after the civil war, Octavian reduced the huge number of the legions (over 60 [24]) to a much more manageable and affordable size (28 [25]). Several legions, particularly those with doubtful loyalties, were simply disbanded. Other legions were amalgamated, a fact suggested by the title Gemina (Twin) [26]. In AD 9 Germanic tribes wiped out three full legions in the Battle of the Teutoburg Forest. This disastrous event reduced the number of the legions to 25. The total of the legions would later be increased again and for the next 300 years always be a little above or below 30 [27].

Augustus also created the Praetorian Guard: nine cohorts ostensibly to maintain the public peace which were garrisoned in Italy. Better paid than the legionaries, the Praetorians also served less time; instead of serving the standard 25 years of the legionaries, they retired after 16 years of service [28].