According to tradition and later writers such as Livy, the Roman Republic was established around 509 BCE, when the last of the seven kings of Rome, Tarquin the Proud, was deposed by Lucius Junius Brutus, and a system based on annually elected magistrates and various representative assemblies was established.

In the 4th century BCE the Republic came under attack by the Gauls, who initially prevailed and sacked Rome. The Romans then took up arms and drove the Gauls back, led by Camillus. The Romans gradually subdued the other peoples on the Italian peninsula, including the Etruscans.

In the 3rd century BCE Rome had to face a new and formidable opponent: the powerful Phoenician city-state of Carthage. In the three Punic Wars, Carthage was eventually destroyed and Rome gained control over Hispania, Sicily and North Africa. After defeating the Macedonian and Seleucid Empires in the 2nd century BCE, the Romans became the dominant people of the Mediterranean Sea.

Towards the end of the 2nd century BCE, a huge migration of Germanic tribes took place, led by the Cimbri and the Teutones. At the Battle of Aquae Sextiae and the Battle of Vercellae the Germans were virtually annihilated, which ended the threat.

In 53 BCE, the Triumvirate disintegrated at Crassus' death. Crassus had acted as mediator between Caesar and Pompey, and, without him, the two generals began to fight for power. After being victorious in the Gallic Wars and earning respect and praise from the legions, Caesar was a clear menace to Pompey, that tried to legally remove Caesar's legions. To avoid this, Caesar crossed the Rubicon River and invaded Rome in 49 BCE, rapidly defeating Pompey. He was murdered in 44 BCE, in the Ides of March by the Liberatores. Caesar's assassination caused political and social turmoil in Rome. Octavian annihilated Egyptian forces in the Battle of Actium in 31 BCE. Mark Antony and Cleopatra committed suicide, leaving Octavianus the sole ruler of the Republic.