The fifteen years following Caesar's assassination were a period of continued instability for Rome, as his successors fought among themselves for supremacy. The Roman people learned a difficult truth—Caesar was not the last tyrant. The Republic was in its last gasps, and if it were to survive it would be dependent on the armies of Brutus and Cassius, but even their victory was uncertain to secure the future of the Republic given all that had happened. In their absence from Rome, Antony, Octavian, and Lepidus formalized their alliance, often called the Second Triumvirate by modern historians. In November 43 BCE, this triumvirate received official status when a law was passed through the assembly of the people granting them a five-year period to put the state in order, which included the authority to make laws, appoint public officials, and allot provinces to governors. There were still elections in Rome, but they were merely affirmations of candidates put forward by the triumvirs, who designated public officials several years in advance, following Caesar's precedent. Even the consuls were now under their authority. One of their first orders of business was to draw up the proscription lists that allowed their opponents, such as Cicero, to be killed without penalty and their property confiscated. Julius Caesar was declared a god by the senate on January 1, 42 BCE, which made Octavian, his adopted son, the son of a god. Caesar's divinization was unprecedented; not since Rome's dim origins had any Roman been made a god, but the Romans would get used to the idea of emperors becoming divine upon their death. Octavian had his own aspirations.

Cassius and Brutus were still in the east with a great many legions, which they had built up over the course of two years. The Triumvirate would need to confront them, which they did at Philippi in the autumn of 42 BCE. Antony and Octavian narrowly emerged victorious. Octavian showed no mercy against his defeated enemies and even had Brutus decapitated, sending his head to Rome as an offering to Caesar. One could argue that the Republic stopped functioning in 49 BCE, but the defeat at Philippi certainly sounded its death knell. The battle of Philippi in 42 BCE represented the final time public armies would fight on behalf of the Republic.