Romulus and Remus, raised by a she-wolf.

According to legend, Rome was founded on April 21, 753 BC by twin descendants of the Trojan prince Aeneas, Romulus and Remus. The Latin King Numitor of Alba Longa was ejected from his throne by his cruel brother Amulius and Numitor's daughter, Rhea Silvia, gave birth to Romulus and Remus. Rhea Silvia was a Vestal Virgin who was raped by Mars, making the twins half-divine. The new king feared that Romulus and Remus would take back the throne, so they were to be drowned. A she-wolf (or a shepherd's wife in some accounts) saved and raised them, and when they were old enough, they returned the throne of Alba Longa to Numitor. The twins then founded their own city, but Romulus killed Remus in a quarrel over which one of them was to reign as the King of Rome. Romulus became the source of the city's name. As the city was bereft of women, legend says that the Latins invited the Sabines to a festival and stole their unmarried maidens, leading to the integration of the Latins and the Sabines.

The city of Rome grew from settlements around a ford on the river Tiber, a crossroads of traffic and trade. According to archaeological evidence, the village of Rome was probably founded sometime in the 8th century BC, though it may go back as far as the 10th century BC, by members of the Latin tribe of Italy, on the top of the Palatine Hill. The Etruscans, who had previously settled to the north in Etruria, seem to have established political control in the region by the late 7th century BC, forming the aristocratic and monarchial elite. The Etruscans apparently lost power in the area by the late 6th century BC, and at this point, the original Latin and Sabine tribes reinvented their government by creating a republic, with much greater restraints on the ability of rulers to exercise power.

The Roman Republic was established around 509 BC, according to later writers such as Livy, when the last of the seven kings of Rome, Tarquin the Proud, was deposed, and a system based on annually elected magistrates and various representative assemblies was established. A constitution set a series of checks and balances, and a separation of powers. The most important magistrates were the two consuls, who together exercised executive authority in the form of imperium, or military command. The consuls had to work with the senate, which was initially an advisory council of the ranking nobility, or patricians, but grew in size and power over time. Other magistracies in the Republic include praetors, aediles, and quaestors. The magistracies were originally restricted to patricians, but were later opened to common people, or plebeians. Republican voting assemblies included the comitia centuriata (centuriate assembly), which voted on matters of war and peace and elected men to the most important offices, and the comitia tributa (tribal assembly), which elected less important offices.

The Romans gradually subdued the other peoples on the Italian peninsula, including the Etruscans. The last threat to Roman hegemony in Italy came when Tarentum, a major Greek colony, enlisted the aid of Pyrrhus of Epirus in 281 BC, but this effort failed as well. The Romans secured their conquests by founding Roman colonies in strategic areas, establishing stable control over the region. In the second half of the 3rd century BC, Rome clashed with Carthage in the first of three Punic Wars. These wars resulted in Rome's first overseas conquests, of Sicily and Hispania, and the rise of Rome as a significant imperial power. After defeating the Macedonian and Seleucid Empires in the 2nd century BC, the Romans became the dominant people of the Mediterranean Sea.

Foreign dominance led to internal strife. Senators became rich at the provinces' expense, but soldiers, who were mostly small-scale farmers, were away from home longer and could not maintain their land, and the increased reliance on foreign slaves and the growth of latifundia reduced the availability of paid work. Income from war booty, mercantilism in the new provinces, and tax farming created new economic opportunities for the wealthy, forming a new class of merchants, the equestrians. The lex Claudia forbade members of the Senate from engaging in commerce, so while the equestrians could theoretically join the Senate, they were severely restricted in terms of political power. The Senate squabbled perpetually, repeatedly blocking important land reforms and refusing to give the equestrian class a larger say in the government. Violent gangs of the urban unemployed, controlled by rival Senators, intimidated the electorate through violence. The situation came to a head in the late 2nd century BC under the Gracchi brothers, a pair of tribunes who attempted to pass land reform legislation that would redistribute the major patrician landholdings among the plebeians. Both brothers were killed, but the Senate passed some of their reforms in an attempt to placate the growing unrest of the plebeian and equestrian classes. The denial of Roman citizenship to allied Italian cities led to the Social War of 91–88 BC. The military reforms of Gaius Marius resulted in soldiers often having more loyalty to their commander than to the city, and a powerful general could hold the city and Senate ransom. This led to civil war between Marius and his protegé Sulla, and culminated in Sulla's dictatorship of 81–79 BC.

In the mid-1st century BC, three men, Julius Caesar, Pompey, and Crassus, formed a secret pact—the First Triumvirate—to control the Republic. After Caesar's conquest of Gaul, a stand-off between Caesar and the Senate led to civil war, with Pompey leading the Senate's forces. Caesar emerged victorious, and was made dictator for life. In 44 BC, Caesar was assassinated by senators who opposed Caesar's assumption of absolute power and wanted to restore constitutional government, but in the aftermath a Second Triumvirate, consisting of Caesar's designated heir, Octavian, and his former supporters, Mark Antony and Lepidus, took power. However, this alliance soon descended into a struggle for dominance. Lepidus was exiled, and when Octavian defeated Antony and Cleopatra of Egypt at the Battle of Actium in 31 BC, he became the undisputed ruler of Rome.

With his enemies defeated, Octavian took the name Augustus and assumed almost absolute power, retaining only a pretense of the Republican form of government. His designated successor, Tiberius, took power without serious opposition, establishing the Julio-Claudian dynasty, which lasted until the death of Nero in 68. The territorial expansion of what was now the Roman Empire continued, and the state remained secure, despite a series of emperors widely viewed as depraved and corrupt (for example, Caligula is argued by some to have been insane and Nero had a reputation for cruelty and being more interested in his private concerns than the affairs of the state). Their rule was followed by the Flavian dynasty. During the reign of the "Five Good Emperors" (96–180), the Empire reached its territorial, economic, and cultural zenith. The state was secure from both internal and external threats, and the Empire prospered during the Pax Romana ("Roman Peace"). With the conquest of Dacia during the reign of Trajan, the Empire reached the peak of its territorial expansion; Rome's dominion now spanned 2.5 million square miles (6.5 million km²).

The period between 193 and 235 was dominated by the Severan dynasty, and saw several incompetent rulers, such as Elagabalus. This and the increasing influence of the army on imperial succession led to a long period of imperial collapse and external invasions known as the Crisis of the Third Century. The crisis was ended by the more competent rule of Diocletian, who in 293 divided the Empire into an eastern and western half ruled by a tetrarchy of two co-emperors and their two junior colleagues. The various co-rulers of the Empire competed and fought for supremacy for more than half a century. On May 11, 330, Emperor Constantine I firmly established Byzantium as the capital of the Roman Empire and renamed it Constantinople. The Empire was permanently divided into the Eastern Roman Empire (later known as the Byzantine Empire) and the Western Roman Empire in 395.

The Western Empire was constantly harassed by barbarian invasions, and the gradual decline of the Roman Empire continued over the centuries. In the 4th century, the westward migration of the Huns caused the Visigoths to seek refuge within the borders of the Roman Empire. In 410, the Visigoths, under the leadership of Alaric I, sacked the city of Rome itself. The Vandals invaded Roman provinces in Gaul, Spain, and northern Africa, and in 455 sacked Rome. On September 4, 476, the Germanic chief Odoacer forced the last Roman emperor in the west, Romulus Augustus, to abdicate. Having lasted for approximately 1200 years, the rule of Rome in the West came to an end.

The Eastern Empire, by contrast, would suffer a similar fate, though not as drastic. Justinian managed to briefly reconquer Northern Africa and Italy, but Byzantine possessions in the West were reduced to southern Italy and Sicily within a few years after Justinian's death. In the east the Byzantines were threatened by the rise of Islam, whose followers rapidly conquered territories in Syria and Egypt and soon presented a direct threat to Constantinople. The Byzantines, however, managed to stop Islamic expansion into their lands during the 8th century, and beginning in the 9th century reclaimed the conquered lands. In 1000 AD the Eastern Empire was at its height: Basileios II reconquered Bulgaria and Armenia, culture and trade flourished. However, soon after the expansion was abruptly stopped in 1071 at the Battle of Manzikert. This finally lead the empire into a dramatic decline. Several centuries of internal strife and Turkic invasions ultimately paved the way for Emperor Alexius I Comnenus to send a call for help to the West in 1095. The West responded with the Crusades, eventually resulting in the Sack of Constantinople by participants in the Fourth Crusade. The conquest of Constantinople in 1204 would see the fragmentation of what little remained of the empire into successor states, the ultimate victor being that of Nicaea. After the recapture of Constantinople by imperial forces, the empire was little more than a Greek state confined to the Aegean coast. The Eastern Empire came to an end when Mehmed II conquered Constantinople on May 29, 1453.