Executive authority was vested in elected officials who served one-year terms and exercised power bounded by the twin principles of annuality and collegiality, whereby each office was held by at least two colleagues who could veto one another's actions. By the second century BC, the Lex Villia Annalis of 180 BC formalized the cursus honorum ("course of offices"), a fixed sequential ladder of magistracies with minimum age requirements and mandatory intervals between terms, following ten years of military service.²²

Entry into the cursus honorum began with the quaestorship, whose holders managed the state treasury (aerarium) and served as financial officers to generals and provincial governors.²³ Above the quaestors stood the aediles—divided between plebeian and curule aediles—who supervised urban maintenance, public markets, the grain supply, and public games. The next rung was the praetorship, the first office to confer imperium, the supreme executive and military authority to command armies and interpret law. The praetor urbanus oversaw civil litigation between citizens in Rome, while the praetor peregrinus handled disputes involving foreigners; additional praetors were added over time to govern newly annexed provinces and preside over permanent criminal courts.²⁴ At the apex stood the two consuls, who served as heads of state, summoned the Senate and assemblies, and led the Republic's chief field armies.

Outside the regular annual sequence stood two specialized offices. Every five years, two censors were elected from among former consuls for an eighteen-month term to conduct the citizen census, assess property qualifications, award public contracts, and regulate public morals by adding or expelling members of the Senate and equestrian order. In times of severe military or domestic emergency, the consuls, acting on the Senate's advice, could appoint a single dictator who wielded supreme imperium assisted by a magister equitum ("master of the horse"), though the dictator's mandate was strictly limited to six months or the resolution of the crisis.²⁵²⁶

The Senate (Senatus), comprising roughly 300 members during the Early and Middle Republic and expanded to 600 in the first century BC, was nominally an advisory council to the magistrates. Because senators held their seats for life and included all former magistrates, the body accumulated immense collective prestige (auctoritas). Although its decrees (senatus consulta) were technically formal advice rather than law, magistrates rarely defied its collective will.²⁷ The Senate directed foreign diplomacy, received ambassadors, assigned military commands and provinces to magistrates, and controlled all state expenditures from the treasury.²⁸

Sovereignty (maiestas) resided formally with the Roman people (Populus Romanus), where adult male citizens—including freedmen—voted in three distinct assemblies to enact laws, declare war, and elect magistrates. Unlike modern representative legislatures, Roman assemblies practiced direct block voting: citizens had to attend in person in Rome, no discussion took place during the voting session itself (debates occurred beforehand in non-voting public meetings called contiones convened by a magistrate), and each voting unit cast a single collective ballot determined by the majority of voters present within that unit.

The oldest body, the Curiate Assembly (comitia curiata), became largely ceremonial under the Republic, ratifying religious matters and formally conferring imperium on newly elected magistrates. The Centuriate Assembly (comitia centuriata), organized originally along military lines into 193 centuriae based on wealth as assessed in the census, elected the senior magistrates (consuls, praetors, and censors), tried high treason cases, and held the sole power to declare war. Because the wealthiest property classes controlled a disproportionate number of centuries and voted first, elections could be decided before poorer citizens cast a ballot. The Tribal Assembly (comitia tributa), which grouped all citizens into thirty-five geographic tribes (four urban and thirty-one rural), elected lower magistrates and became, alongside the patrician-excluded Plebeian Council, the primary vehicle for passing routine legislation.

Following the expulsion of the kings, Rome formed a defensive alliance with its fellow Latin communities through the Foedus Cassianum (493 BC) and spent the fifth century fending off raids by mountain tribes while rivaling the Etruscan city of Veii, which Rome captured after a prolonged siege in 396 BC. Around 390 BC (or 387 BC according to Greek synchronisms), a warband of Senonian Gauls led by Brennus routed the Roman army at the Battle of the Allia and sacked Rome itself. Recovering rapidly, Rome fortified the city with the Servian Wall and reorganized its military, replacing the rigid Greek-style hoplite phalanx with the flexible manipular legion, which divided infantry into three lines of small tactical units (manipuli) armed with heavy javelins (pila) and short swords (gladii).²⁹

Between 343 and 290 BC, Rome fought three grueling Samnite Wars against the confederation of Oscan-speaking tribes in the central Apennines, interspersed with the Latin War (340–338 BC), after which Rome dissolved the Latin League.³⁰ Victory at the Battle of Sentinum in 295 BC over a coalition of Samnites, Etruscans, Umbrians, and Gauls secured Roman dominance in central Italy. When Rome encroached upon the Greek city-states of southern Italy (Magna Graecia), the city of Tarentum appealed to King Pyrrhus of Epirus, who invaded Italy in 280 BC. Despite winning costly battles at Heraclea and Asculum against Roman legions, Pyrrhus could not match Rome's vast manpower reserves; following his strategic defeat at Beneventum in 275 BC, he withdrew to Greece, leaving Rome in control of all peninsular Italy south of the Po Valley.

Rather than annexing all conquered territories or exacting direct tribute, Rome bound the peoples of Italy into a tiered network of bilateral treaties. Some communities received full Roman citizenship, others received "Latin rights" (ius Latii, including commercial and intermarriage rights with Rome), and the majority became autonomous allies (socii) required to supply troops (alae) to fight alongside the citizen legions under Roman command.³¹ This confederation gave the Republic a pool of military manpower unmatched by any contemporary state.

Expansion into southern Italy brought Rome into direct conflict with Carthage, the preeminent Phoenician maritime empire of the western Mediterranean, over control of Sicily. In the First Punic War (264–241 BC), Rome built its first major war fleets and defeated Carthage at sea, compelling it to evacuate Sicily and pay a heavy indemnity; Sicily, soon followed by Sardinia and Corsica, became Rome's first overseas provinces governed by praetors.

Seeking to compensate for these losses, the Carthaginian Barcid family expanded its dominion in Iberia, sparking the Second Punic War (218–201 BC). The Carthaginian general Hannibal crossed the Alps into Italy and inflicted catastrophic defeats on Roman armies at the Trebia, Lake Trasimene, and most famously at Cannae in 216 BC, where upward of 50,000 Roman and allied soldiers perished. Adopting the "Fabian strategy" of attrition pioneered by Quintus Fabius Maximus Verrucosus, Rome refused decisive battle in Italy while holding the loyalty of most of its northern and central Italian allies. Meanwhile, Publius Cornelius Scipio (later Scipio Africanus) expelled Carthaginian forces from Hispania and invaded North Africa, where he defeated Hannibal at the Battle of Zama in 202 BC, stripping Carthage of its empire and reducing it to a client state.

Victory in the west coincided with aggressive intervention in the Hellenistic east, where Philip V of Macedon had allied with Hannibal during the Second Punic War.³² Answering appeals from Greek states such as Pergamum and Rhodes, Rome defeated Philip V at Cynoscephalae in 197 BC during the Second Macedonian War, defeated the Seleucid monarch Antiochus III at Magnesia in 190 BC during the Roman–Seleucid War, and crushed Philip's son Perseus at the Battle of Pydna in 168 BC during the Third Macedonian War, dismantling the Macedonian monarchy.³³ In 146 BC, a watershed year of imperial consolidation, Roman armies under Lucius Mummius razed Corinth and annexed Greece after crushing the Achaean League, while Scipio Aemilianus concluded the Third Punic War (149–146 BC) by utterly destroying Carthage and annexing its home territory as the province of Africa. When King Attalus III of Pergamum bequeathed his wealthy kingdom to the Roman people in 133 BC (organized as the province of Asia), Rome stood without a peer competitor across the Mediterranean.

In 133 BC, the plebeian tribune Tiberius Sempronius Gracchus sought to address the perceived decline of the Italian smallholder class—and the resulting shortage of property-qualified legionary recruits—by proposing the Lex Sempronia Agraria to enforce the 367 BC legal limits on holdings of public land and redistribute the excess to poor citizens. Bypassing the Senate, Tiberius took his bill directly to the Plebeian Council, deposed a fellow tribune who vetoed the measure, and proposed using the newly bequeathed treasury of Pergamum to fund the land allotments. When Tiberius stood for consecutive re-election, a mob of senators led by the Pontifex Maximus Publius Cornelius Scipio Nasica beat him and hundreds of his followers to death with clubs—the first outbreak of lethal political violence in Rome in centuries. A decade later, his younger brother Gaius Gracchus, serving as tribune in 123–122 BC, pursued a far broader program that included state-subsidized grain for citizens (Lex Frumentaria), transferring control of the extortion courts to the equestrians, founding overseas colonies, and proposing citizenship for Italian allies. In 121 BC, the Senate passed the first senatus consultum ultimum ("final decree"), a martial-law resolution authorizing the consul Lucius Opimius to use lethal force against Gaius and his supporters, thousands of whom were killed.

Military failures in the Jugurthine War in Numidia and the Cimbrian War against migrating Germanic tribes propelled the novus homo ("new man") Gaius Marius to the consulship in 107 BC, an office he later held an unprecedented five consecutive times from 104 to 100 BC. In 107 BC, Marius abolished the traditional property requirement for legionary service, openly recruiting landless poor citizens (capite censi) as volunteer professional soldiers and standardizing the legion around the tactical unit of the cohort. Because the Senate routinely refused to establish a systemic pension system for discharged veterans, soldiers became dependent on their individual commanders to secure land grants for them through political pressure, shifting the army's primary loyalty from the Republic to its generals.

Tensions in Italy erupted in 91 BC following the assassination of the tribune Marcus Livius Drusus, who had proposed granting Roman citizenship to the Italian socii. Many Italian communities, led by the Marsi and Samnites, seceded to form a rival confederation called "Italia" with its capital at Corfinium, sparking the devastating Social War (91–87 BC).⁴¹⁴² Rome prevailed militarily only by conceding politically through the Lex Julia (90 BC) and Lex Plautia Papiria (89 BC), which extended full Roman citizenship to all Italian communities south of the Po River that remained loyal or laid down their arms. Immediately thereafter, in 88 BC, a dispute between the consul Lucius Cornelius Sulla and the aged Marius over the lucrative command against King Mithridates VI of Pontus prompted Sulla to take the unprecedented step of marching his six legions directly on the city of Rome. After Marian forces seized the city during Sulla's eastern campaign, Sulla returned in 83 BC, won a bloody civil war at the Battle of the Colline Gate (82 BC), and had himself appointed dictator without a six-month time limit to rewrite the constitution. Sulla instituted the first proscriptions—publishing lists of political enemies who could be murdered for a bounty and whose estates were confiscated—while doubling the Senate to 600, stripping the tribunes of the plebs of their legislative initiative and veto powers, and strictly enforcing the cursus honorum before voluntarily resigning in 79 BC.

Sulla's constitutional settlement unraveled within a decade. In 70 BC, two former Sullan commanders, Gnaeus Pompeius Magnus (Pompey) and Marcus Licinius Crassus, were elected consuls despite Pompey having never held a prior magistracy or sat in the Senate, and they restored the full powers of the plebeian tribunate. During the 60s BC, tribunes passed laws granting Pompey sweeping, multi-province extraordinary imperium to eradicate Mediterranean piracy and defeat Mithridates VI, allowing him to reorganize the Near East and annex Syria. Meanwhile, in 63 BC, the consul Cicero exposed and suppressed the Catilinarian conspiracy, an attempted coup led by the indebted patrician Lucius Sergius Catilina, controversially executing five conspirators without trial under the senatus consultum ultimum.

When the optimates in the Senate—led by Marcus Porcius Cato the Younger—blocked Pompey's eastern settlements, denied land to his veterans, and thwarted the ambitions of Crassus and Gaius Julius Caesar, the three men formed an informal political alliance in 60 BC known to modern historians as the First Triumvirate. Elected consul for 59 BC, Caesar forced through legislation satisfying his partners and secured a five-year proconsular command in Gaul, which he used to conquer all of Gaul up to the Rhine and amass vast wealth and a veteran army. The alliance fractured after the death of Caesar's daughter Julia (Pompey's wife) in 54 BC and the death of Crassus at the Battle of Carrhae against the Parthian Empire in 53 BC. Amid escalating street violence in Rome, the Senate turned to Pompey, appointing him sole consul in 52 BC and demanding that Caesar lay down his command.

In January 49 BC, Caesar defied the Senate by leading his thirteenth legion across the Rubicon river into Italy, igniting a civil war that spanned the Mediterranean. Defeating Pompey at Pharsalus in 48 BC and crushing the remaining senatorial forces in Africa and Spain, Caesar assumed the dictatorship—first for ten years and then, in early 44 BC, in perpetuity (dictator perpetuo). On the Ides of March (15 March) 44 BC, a conspiracy of roughly sixty senators led by Marcus Junius Brutus and Gaius Cassius Longinus assassinated Caesar in the belief that his death would automatically restore the Republic.

Instead, Caesar's lieutenant Mark Antony, Caesar's nineteen-year-old adopted son and heir Octavian, and the general Marcus Aemilius Lepidus formed the Second Triumvirate in 43 BC. Unlike the first, this was a legally constituted magistracy (triumviri rei publicae constituendae) enacted by the Lex Titia, which granted the three men dictatorial powers for five years. The triumvirs launched a new wave of proscriptions that claimed the life of Cicero, defeated Brutus and Cassius at the Battle of Philippi in 42 BC, and partitioned the Roman world among themselves. After sidelining Lepidus in 36 BC, Octavian waged a propaganda and military campaign against Antony and his ally Queen Cleopatra VII of Egypt, defeating their combined fleet at the Battle of Actium in 31 BC and annexing Egypt in 30 BC following their suicides. In January 27 BC, having consolidated unchallenged control over Rome's legions and provinces, Octavian staged a formal "restoration" of power to the Senate and people; in response, the Senate granted him overarching proconsular imperium over the frontier provinces and the revered title Augustus, marking the end of the Roman Republic and the beginning of the Roman Empire.⁴³