Following his death, Caesar's right-hand man, a skilled general named Mark Antony, joined with Octavian and another general named Lepidus to form the Second Triumvirate. In 43 BCE they seized control in Rome and then launched a successful campaign against the old Republican loyalists, killing off the men who had killed Caesar and murdering the strongest senators and equestrians who had tried to restore the old institutions. Mark Antony and Octavian soon pushed Lepidus to the side and divided up control of Roman territory - Octavian taking Europe and Mark Antony taking the eastern territories and Egypt. This was an arrangement that was not destined to last; the two men had only been allies for the sake of convenience, and both began scheming as to how they could seize total control of Rome’s vast empire.
Mark Antony moved to the Egyptian city of Alexandria, where he set up his court. He followed in Caesar’s footsteps by forging both a political alliance and a romantic relationship with Cleopatra, and the two of them were able to rule the eastern provinces of the Republic in defiance of Octavian. In 34 BCE, Mark Antony and Cleopatra declared that Cleopatra’s son by Julius Caesar, Caesarion, was the heir to Caesar (not Octavian), and that their own twins were to be rulers of Roman provinces. Rumors in the west claimed that Antony was under Cleopatra’s thumb (which is unlikely: the two of them were both savvy politicians and seem to have shared a genuine affection for one another) and was breaking with traditional Roman values, and Octavian seized on this behavior to claim that he was the true protector of Roman morality. Soon, Octavian produced a will that Mark Antony had supposedly written ceding control of Rome to Cleopatra and their children on his death; whether or not the will was authentic, it fit in perfectly with the publicity campaign on Octavian’s part to build support against his former ally in Rome.
When he finally declared war in 32 BCE, Octavian claimed he was only interested in defeating Cleopatra, which led to broader Roman support because it was not immediately stated that it was yet another Roman civil war. Antony and Cleopatra’s forces were already fairly scattered and weak due to a disastrous campaign against the Persians a few years earlier. In 31 BCE, Octavian defeated Mark Antony's forces, which were poorly equipped, sick, and hungry. Antony and Cleopatra’s soldiers were starved out by a successful blockade engineered by Octavian and his friend and chief commander Agrippa, and the unhappy couple killed themselves the next year in exile. Octavian was 33. As his grand-uncle had before him, Octavian began the process of manipulating the institutions of the Republic to transform it into something else entirely: an empire.