The Roman Navy (Latin: Classis, lit. "fleet") not only aided in the supply and transport of the legions, but also helped in the protection of the frontiers in the rivers Rhine and Danube. Another of its duties was the protection of the very important maritime trade routes against the threat of pirates. Therefore it patrolled the whole of the Mediterranean Sea, parts of the North Atlantic (coasts of Hispania, Gaul, and Britannia), and had also a naval presence in the Black Sea. Nevertheless the army was considered the senior and more prestigious branch

While the Auxillia (Latin: auxilia = supports) are not as famous as the legionaries, they were of major importance. Unlike the legionaries, the auxilia were recruited from among the non-citizens. Organized in smaller units of roughly cohort strength, they were paid less than the legionaries, and after 25 years of service were rewarded with Roman citizenship, also extended to their sons. According to Tacitus [29] there were roughly as many auxiliaries as there were legionaries. Since at this time there were 25 legions of around 5,000 men each, the auxilia thus amounted to around 125,000 men, implying approximately 250 auxiliary regiments

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].