The early Roman army (c. 500 BC) was, like those of other contemporary city-states influenced by Greek civilization, a citizen militia that practiced hoplite tactics. It was small (the population of free men of military age was then about 9,000) and organized in five classes (in parallel to the comitia centuriata, the body of citizens organized politically), with three providing hoplites and two providing light infantry. The early Roman army was tactically limited and its stance during this period was essentially defensive. By the 3rd century BC, the Romans abandoned the hoplite formation in favor of a more flexible system in which smaller groups of 120 (or sometimes 60) men called maniples could maneuver more independently on the battlefield. Thirty maniples arranged in three lines with supporting troops constituted a legion, totaling between 4,000 and 5,000 men.

The early Republican legion consisted of five sections, each of which was equipped differently and had different places in formation: the three lines of manipular heavy infantry (hastati, principes and triarii), a force of light infantry (velites), and the cavalry (equites). With the new organization came a new orientation toward the offensive and a much more aggressive posture toward adjoining city-states. At nominal full strength, an early Republican legion included 4,000 to 5,000 men: 3,600 to 4,800 heavy infantry, several hundred light infantry, and several hundred cavalrymen. Legions were often significantly understrength from recruitment failures or following periods of active service due to accidents, battle casualties, disease and desertion. During the Civil War, Pompey's legions in the east were at full strength because they were recently recruited, while Caesar's legions were often well below nominal strength after long active service in Gaul. This pattern also held true for auxiliary forces.

Until the late Republican period, the typical legionary was a property-owning citizen farmer from a rural area (an adsiduus) who served for particular (often annual) campaigns, and who supplied his own equipment and, in the case of equites, his own mount. Harris suggests that down to 200 BC, the average rural farmer (who survived) might participate in six or seven campaigns. Freedmen and slaves (wherever resident) and urban citizens did not serve except in rare emergencies. After 200 BC, economic conditions in rural areas deteriorated as manpower needs increased, so that the property qualifications for service were gradually reduced. Beginning with Gaius Marius in 107 BC, citizens without property and some urban-dwelling citizens (proletarii) were enlisted and provided with equipment, although most legionaries continued to come from rural areas. Terms of service became continuous and long—up to twenty years if emergencies required it although Brunt argues that six- or seven-year terms were more typical.

Beginning in the 3rd century BC, legionaries were paid stipendium (amounts are disputed but Caesar famously "doubled" payments to his troops to 225 denarii a year), could anticipate booty and donatives (distributions of plunder by commanders) from successful campaigns and, beginning at the time of Marius, often were granted allotments of land upon retirement. Cavalry and light infantry attached to a legion (the auxilia) were often recruited in the areas where the legion served. Caesar formed a legion, the Fifth Alaudae, from non-citizens in Transalpine Gaul to serve in his campaigns in Gaul. By the time of Caesar Augustus, the ideal of the citizen-soldier had been abandoned and the legions had become fully professional. Legionaries received 900 sesterces a year and could expect 12,000 sesterces on retirement. At the end of the Civil War, Augustus reorganized Roman military forces, discharging soldiers and disbanding legions. He retained 28 legions, distributed through the provinces of the Empire. During the Principate, the tactical organization of the Army continued to evolve. The auxilia remained independent cohorts, and legionary troops often operated as groups of cohorts rather than as full legions. A new versatile type of unit - the cohortes equitatae – combined cavalry and legionaries in a single formation. They could be stationed at garrisons or outposts and could fight on their own as balanced small forces or combine with other similar units as a larger legion-sized force. This increase in organizational flexibility helped ensure the long-term success of Roman military forces.

The Emperor Gallienus (253–268 AD) began a reorganization that created the last military structure of the late Empire. Withdrawing some legionaries from the fixed bases on the border, Gallienus created mobile forces (the Comitatenses or field armies) and stationed them behind and at some distance from the borders as a strategic reserve. The border troops (limitanei) stationed at fixed bases continued to be the first line of defense. The basic unit of the field army was the "regiment", legiones or auxilia for infantry and vexellationes for cavalry. Evidence suggests that nominal strengths may have been 1,200 men for infantry regiments and 600 for cavalry, although many records show lower actual troop levels (800 and 400). Many infantry and cavalry regiments operated in pairs under the command of a comes. In addition to Roman troops, the field armies included regiments of "barbarians" recruited from allied tribes and known as foederati. By 400 AD, foederati regiments had become permanently established units of the Roman army, paid and equipped by the Empire, led by a Roman tribune and used just as Roman units were used. In addition to the foederati, the Empire also used groups of barbarians to fight along with the legions as "allies" without integration into the field armies. Under the command of the senior Roman general present, they were led at lower levels by their own officers.

Military leadership evolved greatly over the course of the history of Rome. Under the monarchy, the hoplite armies were led by the kings of Rome. During the early and middle Roman Republic, military forces were under the command of one of the two elected consuls for the year. During the later Republic, members of the Roman Senatorial elite, as part of the normal sequence of elected public offices known as the cursus honorum, would have served first as quaestor (often posted as deputies to field commanders), then as praetor. Following the end of a term as praetor or consul, a Senator might be appointed by the Senate as a propraetor or proconsul (depending on the highest office held before) to govern a foreign province. More junior officers (down to but not including the level of centurion) were selected by their commanders from their own clientelae or those recommended by political allies among the Senatorial elite.

Under Augustus, whose most important political priority was to place the military under a permanent and unitary command, the Emperor was the legal commander of each legion but exercised that command through a legatus (legate) he appointed from the Senatorial elite. In a province with a single legion, the legate commanded the legion (legatus legionis) and also served as provincial governor, while in a province with more than one legion, each legion was commanded by a legate and the legates were commanded by the provincial governor (also a legate but of higher rank). During the later stages of the Imperial period (beginning perhaps with Diocletian), the Augustan model was abandoned. Provincial governors were stripped of military authority, and command of the armies in a group of provinces was given to generals (duces) appointed by the Emperor. These were no longer members of the Roman elite but men who came up through the ranks and had seen much practical soldiering. With increasing frequency, these men attempted (sometimes successfully) to usurp the positions of the Emperors who had appointed them. Decreased resources, increasing political chaos and civil war eventually left the Western Empire vulnerable to attack and takeover by neighboring barbarian peoples.

Less is known about the Roman navy than the Roman army. Prior to the middle of the 3rd century BC, officials known as duumviri navales commanded a fleet of twenty ships used mainly to control piracy. This fleet was given up in 278 AD and replaced by allied forces. The First Punic War required that Rome build large fleets, and it did so largely with the assistance of and financing from allies. This reliance on allies continued to the end of the Roman Republic. The quinquereme was the main warship on both sides of the Punic Wars and remained the mainstay of Roman naval forces until replaced by the time of Caesar Augustus by lighter and more maneuverable vessels. As compared with a trireme, the quinquereme permitted the use of a mix of experienced and inexperienced crewmen (an advantage for a primarily land-based power), and its lesser maneuverability permitted the Romans to adopt and perfect boarding tactics using a troop of about 40 marines in lieu of the ram. Ships were commanded by a navarch, a rank equal to a centurion, who was usually not a citizen. Potter suggests that because the fleet was dominated by non-Romans, the navy was considered non-Roman and allowed to atrophy in times of peace. Information suggests that by the time of the late Empire (350 AD), the Roman navy comprised several fleets including warships and merchant vessels for transportation and supply. Warships were oared sailing galleys with three to five banks of oarsmen. Fleet bases included such ports as Ravenna, Arles, Aquilea, Misenum and the mouth of the Somme River in the West and Alexandria and Rhodes in the East. Flotillas of small river craft (classes) were part of the limitanei (border troops) during this period, based at fortified river harbors along the Rhine and the Danube. That prominent generals commanded both armies and fleets suggests that naval forces were treated as auxiliaries to the army and not as an independent service. The details of command structure and fleet strengths during this period are not well known, although fleets were commanded by prefects.

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 monarchical 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

Roman tradition and archaeological evidence point to a complex within the Forum Romanum as the seat of power for the king and the beginnings of the religious center there as well. Numa Pompilius was the second king of Rome, succeeding Romulus. He began Rome's great building projects with his royal palace the Regia and the complex of the Vestal virgins.

The imperial city of Rome was the largest urban center of its time, with a population of about one million people (about the size of London in the early 19th century, when London was the largest city in the world), with some high-end estimates of 14 million and low-end estimates of 450,000. The public spaces in Rome resounded with such a din of hooves and clatter of iron chariot wheels that Julius Caesar had once proposed a ban on chariot traffic during the day. Historical estimates show that around 20 percent of the population under jurisdiction of ancient Rome (25–40%, depending on the standards used, in Roman Italy) lived in innumerable urban centers, with population of 10,000 and more and several military settlements, a very high rate of urbanization by pre-industrial standards. Most of these centers had a forum, temples, and other buildings similar to those in Rome.

After defeating the Macedonian and Seleucid Empires in the 2nd century BC, the Romans became the dominant people of the Mediterranean Sea.[38][39] The conquest of the Hellenistic kingdoms provoked a fusion between Roman and Greek cultures and the Roman elite, once rural, became a luxurious and cosmopolitan one. By this time Rome was a consolidated empire – in the military view – and had no major enemies.

Gaius Marius, a Roman general and politician who dramatically reformed the Roman military

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 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 and the Senate passed reforms reversing the Gracchi brother's actions. This led to the growing divide of the plebeian groups (populares) and equestrian classes (optimates).

Other magistracies in the Republic include tribunes, quaestors, aediles, praetors and censors. 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.

In the 4th century BC Rome had come under attack by the Gauls. The Gauls until that time had only extended to the Po Valley in the Italian peninsula. The Gauls had been penetrating deep into Etruria, so the Romans decided to join in on the melee. With Etruria completely gone, the Gauls continued their advance south which led them into a fight with the Romans. On 16 July 390 BC, a Gallic army under the leadership of a tribal chieftain named Brennus, met the Romans on the Banks of the small Allia River just ten miles north of Rome. Brennus defeated the Romans, afterwards the Gauls marched directly to Rome. Most Romans had fled the city, those who were capable of fighting barricaded themselves upon the Capitoline Hill for a last stand. The Gauls looted and burned the city, then laid siege to the Capitoline Hill. The siege lasted seven months, the Gauls then agreed to a compromise peace. The Romans were forced to pay the Gauls 1,000 pounds of gold. According to legend, the Roman General supervising the weighing noticed that the Gauls were using false scales. The Romans then took up arms and drove the Gauls back, and then an army led by Camillus defeated the Gauls and he said, "With iron, not with gold, Rome buys her freedom."

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, thereby establishing stable control over the region of Italy.

The class struggles of the Roman Republic resulted in an unusual mixture of democracy and oligarchy. The word republic comes from the Latin res publica, which literally translates to "public business". Roman laws traditionally could only be passed by a vote of the Popular assembly (Comitia Tributa). Likewise, candidates for public positions had to run for election by the people. However, the Roman Senate represented an oligarchic institution, which acted as an advisory body.

In the Republic, the Senate held great authority (auctoritas), but no real legislative power; it was technically only an advisory council. However, as the Senators were individually very influential, it was difficult to accomplish anything against the collective will of the Senate. New Senators were chosen from among the most accomplished patricians by Censors (Censura), who could also remove a Senator from his office if he was found "morally corrupt"; a charge that could include bribery or, as under Cato the Elder, embracing one's wife in public. Later, under the reforms of the dictator Sulla, Quaestors were made automatic members of the Senate, though most of his reforms did not survive.

The Republic had no fixed bureaucracy, and collected taxes through the practice of tax farming. Government positions such as quaestor, aedile, or praefect were funded from the office-holder's private finances. To prevent any citizen from gaining too much power, new magistrates were elected annually and had to share power with a colleague. For example, under normal conditions, the highest authority was held by two consuls. In an emergency, a temporary dictator could be appointed. Throughout the Republic, the administrative system was revised several times to comply with new demands. In the end, it proved inefficient for controlling the ever-expanding dominion of Rome, contributing to the establishment of the Roman Empire. In the early Empire, the pretense of a republican form of government was maintained. The Roman Emperor was portrayed as only a princeps, or "first citizen", and the Senate gained legislative power and all legal authority previously held by the popular assemblies. However, the rule of the Emperors became increasingly autocratic, and the Senate was reduced to an advisory body appointed by the Emperor. The Empire did not inherit a set bureaucracy from the Republic, since the Republic did not have any permanent governmental structures apart from the Senate. The Emperor appointed assistants and advisers, but the state lacked many institutions, such as a centrally planned budget. Some historians have cited this as a significant reason for the decline of the Roman Empire.

The roots of the legal principles and practices of the ancient Romans may be traced to the Law of the Twelve Tables promulgated in 449 BC and to the codification of law issued by order of Emperor Justinian I around 530 AD (see Corpus Juris Civilis). Roman law as preserved in Justinian's codes continued into the Byzantine Empire, and formed the basis of similar codifications in continental Western Europe. Roman law continued, in a broader sense, to be applied throughout most of Europe until the end of the 17th century. The major divisions of the law of ancient Rome, as contained within the Justinian and Theodosian law codes, consisted of Ius Civile, Ius Gentium, and Ius Naturale. The Ius Civile ("Citizen Law") was the body of common laws that applied to Roman citizens. The Praetores Urbani (sg. Praetor Urbanus) were the people who had jurisdiction over cases involving citizens. The Ius Gentium ("Law of nations") was the body of common laws that applied to foreigners, and their dealings with Roman citizens. The Praetores Peregrini (sg. Praetor Peregrinus) were the people who had jurisdiction over cases involving citizens and foreigners. Ius Naturale encompassed natural law, the body of laws that were considered common to all beings.

Roman society is largely viewed as hierarchical, with slaves (servi) at the bottom, freedmen (liberti) above them, and free-born citizens (cives) at the top. Free citizens were also divided by class. The broadest, and earliest, division was between the patricians, who could trace their ancestry to one of the 100 Patriarchs at the founding of the city, and the plebeians, who could not. This became less important in the later Republic, as some plebeian families became wealthy and entered politics, and some patrician families fell on hard times. Anyone, patrician or plebeian, who could count a consul as his ancestor was a noble (nobilis); a man who was the first of his family to hold the consulship, such as Marius or Cicero, was known as a novus homo ("new man") and ennobled his descendants. Patrician ancestry, however, still conferred considerable prestige, and many religious offices remained restricted to patricians.

A class division originally based on military service became more important. Membership of these classes was determined periodically by the Censors, according to property. The wealthiest were the Senatorial class, who dominated politics and command of the army. Next came the equestrians (equites, sometimes translated "knights"), originally those who could afford a warhorse, who formed a powerful mercantile class. Several further classes, originally based on what military equipment their members could afford, followed, with the proletarii, citizens who had no property at all, at the bottom. Before the reforms of Marius they were ineligible for military service and are often described as being just above freed slaves in wealth and prestige.

Voting power in the Republic was dependent on class. Citizens were enrolled in voting "tribes", but the tribes of the richer classes had fewer members than the poorer ones, all the proletarii being enrolled in a single tribe. Voting was done in class order and stopped as soon as most of the tribes had been reached, so the poorer classes were often unable even to cast their votes.

Women shared some basic rights with their male counterparts, but were not fully regarded as citizens and were thus not allowed to vote or take part in politics. At the same time the limited rights of women gradually were expanded (due to emancipation) and women reached freedom from paterfamilias, gained property rights and even had more juridical rights than their husbands, but still they had no voting rights and were absent from politics.

Allied foreign cities were often given the Latin Right, an intermediary level between full citizens and foreigners (peregrini), which gave their citizens rights under Roman law and allowed their leading magistrates to become full Roman citizens. While there were varying degrees of Latin rights, the main division was between those cum suffragio ("with vote"; enrolled in a Roman tribe and able to take part in the comitia tributa) and sine suffragio ("without vote"; could not take part in Roman politics). Some of Rome's Italian allies were given full citizenship after the Social War of 91–88 BC, and full Roman citizenship was extended to all free-born men in the Empire by Caracalla in 212.

After Constantine’s rule, the empire entered a critical stage which terminated with the fall of the Western Roman Empire. Christianity, corruption, and many other factors have been blamed for its decline.

Under the last of the Constantinians and the Valentinian dynasty, Rome lost decisive battles against the Persians and Germanic barbarians: in 363, emperor Julian the Apostate was killed in the Battle of Samarra, against the Persians and the Battle of Adrianople cost the life of emperor Valens (364–378); the victorious Goths were never expelled from the Empire nor assimilated. Theodosius (379–395) gave even more force to the Christian faith; after his death, the Empire was divided into the Eastern Roman Empire, ruled by Arcadius and the Western Roman Empire, commanded by Honorius; both were Theodosius’ sons.

The situation became more critical in 408, after the death of Stilicho, a general who tried to reunite the Empire and repel barbarian invasion in the early years of the 5th century. The professional field army collapsed. In 410, the Theodosian dynasty saw the Visigoths sack Rome. During the 5th century, the Western Empire saw a significant reduction of its territory. The Vandals conquered North Africa, the Visigoths claimed Gaul, Hispania was taken by the Suebi, Britain was abandoned by the central government, and the Empire suffered further from the invasions of Attila, chief of the Huns.

Finally, general Orestes refused to meet the demands of the barbarian “allies” who now formed the army, and tried to expel them from Italy. Unhappy with this, their chieftain Odoacer defeated and killed Orestes, invaded Ravenna and dethroned Romulus Augustus, son of Orestes. This event happened in 476, and historians usually take it as the mark of the end of Classical Antiquity and beginning of the Middle Ages.

According to the founding myth of Rome, the city was founded on 21 April 753 BC by twin brothers Romulus and Remus, who descended from the Trojan prince Aeneas and who were grandsons of the Latin King, Numitor of Alba Longa. King Numitor was deposed from his throne by his brother, Amulius, while Numitor's daughter, Rhea Silvia, gave birth to the twins. Because Rhea Silvia was raped and impregnated by Mars, the Roman god of war, the twins were considered half-divine.

The new king feared Romulus and Remus would take back the throne, so he ordered them 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 the location of the Roman Kingdom, though some sources state the quarrel was about who was going to rule or give his name to the city. Romulus became the source of the city's name. In order to attract people to the city, Rome became a sanctuary for the indigent, exiled, and unwanted. This caused a problem for Rome, which had a large workforce but was bereft of women. Romulus traveled to the neighboring towns and tribes and attempted to secure marriage rights but as Rome was so full of undesirables they all refused. 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.

Another legend, recorded by Greek historian Dionysius of Halicarnassus, says that Prince Aeneas led a group of Trojans on a sea voyage to found a new Troy, since the original was destroyed in the outcome of the Trojan War. After a long time in rough seas, they landed at the banks of the Tiber River. Not long after they landed, the men wanted to take to the sea again, but the women who were traveling with them did not want to leave. One woman, named Roma, suggested that the women burn the ships out at sea to prevent them from leaving. At first, the men were angry with Roma, but they soon realized that they were in the ideal place to settle. They named the settlement after the woman who torched their ships.