Augustus was born into an old and wealthy equestrian branch of the plebeian Julia gens. He was the great-nephew and adopted son of Julius Caesar, who named Octavian as his heir after the murder of Caesar's only legitimate child.
After the assassination of Julius Caesar in 44 BC, the Senate declared war against the assassins, Marcus Junius Brutus and Gaius Cassius Longinus. Octavian allied with Mark Antony and Marcus Aemilius Lepidus in the Second Triumvirate to avenge Caesar's death. Defeating Brutus and Cassius at the Battle of Philippi in 42 BC, the triumvirs divided the Roman world among themselves and proscribed thousands of their political enemies. After securing the east, Antony settled in Alexandria with the Ptolemaic queen, Cleopatra, while Octavian remained in Rome.
The triumvirs' alliance soon soured. In a struggle for sole control of the Roman world, Octavian first launched a propaganda war against Antony and Cleopatra, portraying them as dissolute monarchists who were destroying traditional Roman values. The final conflict took place at the Battle of Actium in 31 BC, where Octavian's forces led by Marcus Vipsanius Agrippa decisively defeated Antony's and Cleopatra's fleet. After failing to capture Cleopatra alive, Octavian secured his victory over Egypt and became the sole master of the Roman world.
Octavian restored the Roman Republic in theory but became its de facto leader under the title Princeps Civitatis ("First Citizen") and became known as Augustus in 27 BC. Throughout his reign, Augustus secured peace within the empire and maintained peace with Parthia by making concessions to gain back the standards lost at the Battle of Carrhae. He reformed the Roman political and military system, created a professional standing Roman army, and encouraged the growth of an imperial cult. His principate saw a great cultural flourishing, which is often referred to as the Pax Romana or the Augustan Age.
On his death in 14 AD, Augustus was deified and succeeded by his stepson, Tiberius. Augustus's rule marked the beginning of the Pax Romana, a period of relative peace and prosperity that lasted for over two centuries.