The powers of an emperor, (his imperium) existed, in theory at least, by virtue of his "tribunician powers" (potestas tribunicia) and his "proconsular powers" (imperium proconsulare). In theory, the tribunician powers (which were similar to those of the Plebeian Tribunes under the old republic) made the emperor's person and office sacrosanct, and gave the emperor authority over Rome's civil government, including the power to preside over and to control the Senate.
The proconsular powers (similar to those of military governors, or Proconsuls, under the old republic) gave him authority over the Roman army. He was also given powers that, under the republic, had been reserved for the Senate and the assemblies, including the right to declare war, to ratify treaties, and to negotiate with foreign leaders.
The emperor also had the authority to carry out a range of duties that had been performed by the censors, including the power to control senate membership. In addition, the emperor controlled the religious institutions, since, as emperor, he was always Pontifex Maximus and a member of each of the four major priesthoods. While these distinctions were clearly defined during the early empire, eventually they were lost, and the emperor's powers became less constitutional and more monarchical.
Realistically, the main support of an emperor's power and authority was the military. Being paid by the imperial treasury, the legionaries also swore an annual military oath of loyalty towards him, called the Sacramentum.
The death of an emperor led to a crucial period of uncertainty and crisis. In theory the senate was entitled to choose the new emperor, but most emperors chose their own successors, usually a close family member. The new emperor had to seek a swift acknowledgement of his new status and authority in order to stabilize the political landscape. No emperor could hope to survive, much less to reign, without the allegiance and loyalty of the Praetorian Guard and of the legions. To secure their loyalty, several emperors paid the donativum, a monetary reward.
While the Roman assemblies continued to meet after the founding of the empire, their powers were all transferred to the Roman Senate, and so senatorial decrees (senatus consulta) acquired the full force of law.
In theory, the emperor and the senate were two equal branches of government, but the actual authority of the senate was negligible and it was largely a vehicle through which the emperor disguised his autocratic powers under a cloak of republicanism. Still prestigious and respected, the Senate was largely a glorified rubber stamp institution which had been stripped of most of its powers, and was largely at the emperor's mercy.
Many emperors showed a certain degree of respect towards this ancient institution, while others were notorious for ridiculing it. During senate meetings, the emperor sat between the two consuls, and usually acted as the presiding officer. Higher ranking senators spoke before lower ranking senators, although the emperor could speak at any time. By the third century, the senate had been reduced to a glorified municipal body.
Senators and equestrians
No emperor could rule the empire without the Senatorial order and the Equestrian order. Most of the more important posts and offices of the government were reserved for the members of these two aristocratic orders. It was from among their ranks that the provincial governors, legion commanders, and similar officials were chosen.
These two classes were hereditary and mostly closed to outsiders. Very successful and favoured individuals could enter, but this was a rare occurrence. The careers of the young aristocrats was influenced by their family connections and the favour of patrons. As important as ability, knowledge, skill, or competence; patronage was considered vital for a successful career and the highest posts and offices required the emperor's favour and trust.
The son of a senator was expected to follow the Cursus honorum, a career ladder, and the more prestigious positions were restricted to senators only. A senator also had to be wealthy; one of the basic requirements was the wealth of 12,000 gold aurei (about 100 kg of gold), a figure which would later be raised with the passing of centuries.
Below the Senatorial order was the Equestrian order. The requirements and posts reserved for this class, while perhaps not so prestigious, were still very important. Some of the more vital posts, like the governorship of Aegyptus, were even forbidden to the members of the Senatorial order and available only to equestrians.
During and after the civil war, Octavian reduced the huge number of the legions (over 60) to a much more manageable and affordable size (28). Several legions, particularly those with doubtful loyalties, were simply disbanded. Other legions were amalgamated, a fact suggested by the title Gemina (Twin)
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.
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.
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 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.
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.