Prior to 509 BC, Rome was a kingdom ruled by kings who were probably of Etruscan origin. The overthrow of these kings came circa 509 BC...

(paraphrased from wikipedia)
Livy's version of the foundation of the Republic states that the last King of Rome, Lucius Tarquinius Superbus ("Tarquin the proud") had a son, Sextus Tarquinius, who raped a Roman noblewoman named Lucretia. Lucretia compelled her family to take action by gathering her kinsmen to tell them what had happened before killing herself. The incident led to an uprising which expelled the Tarquins, royal house, out of Rome into refuge in Etruria.

When a king left office, his powers returned to the senate until a new king was elected. However, Tarquin was so despised that the senate refused to elect a new king in his stead. Instead, it retained his powers, appointing magistrates to exercise them. Lucretia's widowed husband Lucius Tarquinius Collatinus and her brother Lucius Junius Brutus were elected as the first two consuls of the new Republic.

Around 60 BC, the First Triumvirate was formed, comprised of Pompey (Gnaeus Pompeius Magnus), (Marcus Licinius)Crassus and Julius Caesar.

After his invasion of Britain and the collapse of the first triumvirate, Gaius Julius Caesar began a civil war in 49 BC from which he became the undisputed master of the Roman world.

After assuming control, he began extensive reforms of government and society. He was proclaimed dictator for life (dictator perpetuus), and heavily centralized the bureaucracy of the Republic. A group of senators led by Caesar's former friend Marcus Junius Brutus assassinated the dictator on the Ides of March (March 15) in 44 BC, hoping to restore normalcy to the Republic. Unfortunately, the result was another Roman civil war, which ultimately led to the establishment of a permanent autocracy by Caesar's adopted heir, Gaius Octavianus (later "Augustus"). In 42 BC, two years after his assassination, the Senate officially sanctified Caesar as one of the Roman deities.

Adopted as a youth by Julius Caesar, Gaius Octavianus ("Octavian", "Augustus")ruled from 27 BC until his death in AD 14. He came into his inheritance after Caesar's assassination in 44 BC. In 43 BC, Octavian joined forces with Mark Antony and Marcus Aemilius Lepidus in a military dictatorship known as the Second Triumvirate.

Augustus' reign initiated an era of relative peace known as the Pax Romana, literally "Roman peace". Discounting continuous frontier wars, and one year-long civil war over imperial succession, the Mediterranean world remained at peace for over two centuries.

Augustus expanded the empire's boundaries, secured borders with client states, and made peace with Parthia through diplomacy. He reformed the Roman tax system, developed networks of roads with an official courier system, established a standing army (and small navy), established the Praetorian Guard, and created official police and fire-fighting forces. Much of the city was rebuilt under Augustus, who wrote a record of his own accomplishments, the Res Gestae Divi Augusti, which still survives. Upon his death in AD 14, Augustus was declared a god by the Senate. The names Augustus and Caesar were adopted by every subsequent emperor, and the month of Sextilis was officially renamed August in his honour. He was succeeded by his stepson Tiberius.

The Julio-Claudian Dynasty continued until the death of Nero in 68 A.D.

The Roman Empire was hungry for more land, more peoples to enslave, and more wealth from those peoples. At its height in 117 AD, the boundaries stretched from near the Scottish Border in the north to well into Egypt in the south; from the Spanish coast in the west to Turkey and parts of Arabia in the east.

The Romans left their mark on the world in many ways, from architectural styles and engineering innovations to language and government models.