Octavian's victory enabled him to consolidate his power over Rome and its dominions. He adopted the title of Princeps ("first citizen") and some years later was awarded the title of Augustus ("revered") by the Roman Senate. This became the name by which he was known in later times. As Augustus, he retained the trappings of a restored Republican leader, but historians generally view this consolidation of power and the adoption of these honorifics as the end of the Roman Republic and the beginning of the Roman Empire.
The alliance among Octavian, Mark Antony and Marcus Lepidus, commonly known as the Second Triumvirate, was renewed for a five-year term in 38 BC. However, the triumvirate broke down when Octavian saw Caesarion, the professed son of Julius Caesar and Queen Cleopatra VII of Egypt, as a major threat to his power. This occurred when Mark Antony, the other most influential member of the triumvirate, abandoned his wife, Octavian's sister Octavia Minor. Afterwards he moved to Egypt to start a long-term romance with Cleopatra, becoming the de facto stepfather to Caesarion. Such an affair was doomed to become a political scandal. Antony was inevitably perceived by Octavian and the majority of the Roman Senate as the leader of a separatist movement that threatened to break the unity of the Roman Republic.
Octavian's prestige and, more importantly, the loyalty of his legions had been initially boosted by Julius Caesar's legacy of 44 BC, by which 19-year-old Octavian was officially adopted as Caesar's only son and the sole legitimate heir of his enormous wealth. Antony had been the most important and most successful senior officer in Caesar's army (magister equitum) and, thanks to his military record, claimed a substantial share of the political support of Caesar's soldiers and veterans. Both Octavian and Antony had fought against their common enemies in the civil war that followed the assassination of Caesar.
After years of loyal cooperation with Octavian, Antony started to act independently, eventually arousing his rival's suspicion that he was vying to become sole master of Rome. When he left Octavia Minor and moved to Alexandria to become Cleopatra's official partner, he led many Roman politicians to believe that he was trying to become the unchecked ruler of Egypt and other eastern kingdoms while still maintaining his command over the many Roman legions in the East. As a personal challenge to Octavian's prestige, Antony tried to get Caesarion accepted as a true heir of Caesar, even though the legacy did not mention him. Antony and Cleopatra formally elevated Caesarion, then 13, to power in 34 BC, giving him the vague but alarming title of "King of the Kings" (Donations of Alexandria). Being a son of Caesar, such an entitlement was felt as a threat to Roman republican traditions. It was widely believed that Antony had once offered a diadem to Caesar. Thereafter, Octavian started a propaganda war, denouncing Antony as an enemy of Rome, asserting that he was seeking to establish a personal monarchy over the entire Roman Empire on behalf of Caesarion, circumventing the Roman Senate. It was also said that Antony intended to move the capital of the empire to Alexandria.
As the Second Triumvirate formally expired on the last day of 33 BC, Antony wrote to the Senate that he did not wish to be reappointed. He hoped that he might be regarded by them as their champion against the ambition of Octavian, whom he presumed would not be willing to abandon his position in a similar manner. The causes of mutual dissatisfaction between the two had been accumulating. Antony complained that Octavian had exceeded his powers in deposing Lepidus, in taking over the countries held by Sextus Pompeius and in enlisting soldiers for himself without sending half to him. Octavian complained that Antony had no authority to be in Egypt; that his execution of Sextus Pompeius was illegal; that his treachery to the king of Armenia disgraced the Roman name; that he had not sent half the proceeds of the spoils to Rome according to his agreement; and that his connection with Cleopatra and the acknowledgment of Caesarion as a legitimate son of Caesar were a degradation of his office and a menace to himself.