Brutus and Octavian dominated the Republic in the years after Mark Antony’s death, commanding the fierce loyalty of their respective legions, and representing the main two parties, the Optimates and the Caesarean faction. Since Octavian was Caesar’s surviving heir, and Brutus also had looked to Caesar as a father, the two parties had good relations on this basis, and so the Republic enjoyed peace
The Senate too, contented itself with the knowledge that neither man could become too powerful, and so the basis of a Republic could be preserved, there being no single man of supreme power. Brutus in particular, always deferred to the Senate, and restrained Octavian to do the same, as he would stand to lose much (aristocratic support), if he did otherwise.
Octavian attempted to circumvent this with popular support, by proposing populist policies which always got voted down by the Senate, but this tactic was increasingly ineffective. During a time of peace and prosperity which lifted the pockets of the common folk, they had far fewer economic frustrations to vent, and respect for Republican institutions like the Senate grew. Among many plebeians, Brutus was held in roughly equal esteem to Octavian due to his historied family name, and having shared in his accomplishments, such as the defeat of Mark Antony and the seductress witch Cleopatra.
Brutus’ death in 12 BC changed matters dramatically. Octavian moved quickly to secure the loyalty of Brutus’ legions through bribes, from money that he had accumulated throughout his campaigns, especially Egypt. His control being absolute, soon the Senate could no longer deny Octavian supremacy and de facto imperium, there being no other equally renowned statesmen of equal stature to challenge him.
His actions brought about the wrath of the Optimates, who having no other means to challenge him, planned for his assassination. The plot was betrayed however by one of Octavian’s many informers in the Senate and the conspirator’s attempt to relay their intentions to Brutus’ former legions, and Octavian took the opportunity to purge most of the Optimates, sending them into exile and appropriating all their property. With this done, the Senate passed the first settlement, awarding Octavian, now Augustus with a wide range of new, official powers.
Still Augustus was wise enough to rule in a cautious, restrained manner, neither using his powers to engage in outward tyranny or excesses, nor did he punish those who dissented against him using speech, nor did he implement radical policies that would have alienated the Senate. Instead policies like instituting a Cursus Publicus, new infrastructure works in the provinces, especially roads, and a new police and firefighting service for Rome, met with general support.
His foreign policy was cautious, and there were no new attempts to expand the empire, with the exception of southern Germany, where Augustus sent his intended successor, Drusus to defeat the Germanic tribes, and established two new provinces, Raetia and Noricum. This was done to establish a more effective buffer for Italy to protect it. But there was absolutely no attempt to recover the lost province of Caesar, Mesopotamia from Parthia for example. Augustus also demanded and got the submission of Libyan tribes to become a client state of Rome, although he had to wage a short war to subjugate them, and conceded some desert lands traditionally belonging to Egypt to Libyan rule. Rome also got four new client states in the Pontos Euxeinos, greek city states of Tyras, Nikonion, Ordesus, Olbia, all of whom became client states in exchange for legal rights given to their merchants when doing business with Rome, and to secure Roman protection against Scythian raiders.
This paved the way for a second settlement in 2 BC which increased Augustus’ powers. Augustus was careful and subtle enough to make sure that this gradual slide and transition to absolute rule, would appear almost unnoticeable and completely non-alarming to the general public and to Senators.
Augustus accumulated vast wealth in his life due to his control of imperial provinces, provinces that had been established out of former client states of Rome, which were never put under Senatorial control. While Augustus had shared these conquests with Brutus, with Brutus gaining Numidia, Asia Minor, Taurica, Thracia, Cyprus and Augustus gaining Cyrenaica, Egypt, Judea, Syria, and Mauretania. Brutus’ death and the subsequent shift in loyalties of the legions in these provinces, led to Augustus appointing military governors directly for all these provinces, as they were nominally still under military administration.
This vast wealth led to connections within the Senate, the opportunities of patronage, blackmail, bribery, subtle threats of force were enough to cow the Senate into making concessions. In exchange Augustus never attempted to make himself dictator, though he had absolute power in most matters of the Republic and was called Princeps, an extension of the title of princeps senatus. So the outward facade of the Republic was maintained, along with all of the Senate’s gravitas and authority, which satisfied most senators.
Augustus died in AD 9, leaving his vast estate to Drusus. Drusus, being a very capable commander, had already won the respect of the legions in recent years during his campaigns in Germania. This allowed him to maintain control of Augustus’ provinces, as well as most of his troops. So all the ingredients were there, for the principate that Augustus had established, to essentially continue on.