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Military

{{RomanMilitary}} {{Main|Structural history of the Roman military|Early Roman army|Roman army of the mid-Republic|Roman army of the late Republic}} During the Republic, Rome's army--its tactics and structure--evolved dramatically. Rome's military secured Rome's territory and borders, and helped to impose tribute on conquered peoples. Rome's armies had a formidable reputation; but Rome also "produced [its] share of incompetents" and catastrophic defeats. Nevertheless, it was generally the fate of Rome's greatest enemies, such as Pyrrhus and Hannibal, to win early battles but lose the war.

Structure

At the time of the overthrow of the kingdom, military service was considered one's civic duty and Rome's army was composed entirely of citizen-soldiers. The Army consisted of a single legion, primarily of heavily-armored infantry that fought in a Greek hoplite or phalanx formation, which may have been learned from the Etruscans. The main force was supported by light infantry and cavalry. Each soldier was responsible for their own equipment, with the better equipped and armored in the front and the lessor toward the rear. As a result, Rome's front-line soldiers were composed of the wealthier plebes and its light infantry was made-up of the poorer. Roman cavalry was recruited from the patrician class. With the founding of the Republic, the single legion, formerly under the command of the king, was divided in two with each half placed under the command of the one of the two consuls.

By the late 4th century BC, the army had abandoned its hoplite formation, for the "manipular legion", the ranks of which were determined by age and experience, instead of the relative wealth of individual soldiers. The front line soldiers were the youngest and least experienced, while the oldest, most veteran soldiers were at the back.

The maniples were typically deployed in three lines:

• 1. Each first line maniple were leather-armoured infantry soldiers who wore a bronze breastplate and a bronze helmet adorned with 3 feathers approximately 30 cm (12 in) in height and carried an iron-clad wooden shield. They were armed with a sword and two throwing spears.
• 2. The second infantry line was armed and armoured in the same manner as was the first infantry line. The second infantry line, however, wore a lighter coat of mail rather than a solid brass breastplate.
• 3. The third infantry line was the last remnant of the hoplite-style (the Greek-style formation used occasionally during the early Republic) troops in the Roman army. They were armed and armoured in the same manner as were the soldiers in the second line, with the exception that they carried a lighter spear.

The extraordinary demands of the Punic Wars, in addition to a shortage of manpower, exposed the tactical weaknesses of the manipular legion, at least in the short term. In 217 BC, near the beginning of the Second Punic War, Rome was forced to effectively ignore its long-standing principle that its soldiers must be both citizens and property owners. During the 2nd century BC, Roman territory saw an overall decline in population, partially due to the huge losses incurred during various wars. This was accompanied by severe social stresses and the greater collapse of the middle classes.

At the end of the 2nd century BC, under the Consulate of Gaius Marius, major changes to the military completely overhauled its structure. The most significant change of the Marian reforms was the acceptance of landless Romans into the army. Previously, only those who owned property could serve. A pension, in the form of a land grant was offered to recruits. The army became an attractive career path to Rome's poor and the its ranks grew. The Republic now began to outfit these new recruits with standardized equipment and supplies. This led to greater field mobility and less costs by reducing the army's "baggage train".

Now, Rome had a standing army of professional soldiers enlisting for a term of years and granted a pension and land upon completion. Instead of an army, raised annually, or hastily-assembled in times of national emergency, troops could now train and drill during peacetime to prepare for the next war. Rome also passed legislation that offered citizenship to the citizens of Rome's Italian allies in exchange for serving.

Rome's reliance on troops furnished by allies, such as the Etruscans, Umbrians, Apulians, Campanians, Samnites, Lucani, Bruttii, and southern Greek cities, was a fundamental part of its military traditions. In 493, the Treaty of Cassius, created a mutual defense pact between Rome and the socii, the neighboring Latin people. Non-Roman citizens typically made up half and sometimes as much as two-thirds of the forces under Roman command.

A small navy had operated at a fairly low level after about 300 BC, but it was massively upgraded during the First Punic War. After a period of frenetic construction, the navy mushroomed to a size of more than 400 ships on the Carthaginian ("Punic") pattern. Once completed, it could accommodate up to 100,000 sailors and embarked troops for battle. The navy thereafter declined in size. Under Caesar, an invasion fleet was assembled in the English Channel to allow the invasion of Britannia; under Pompey, a large fleet was raised in the Mediterranean Sea to clear the sea of Cilician pirates. During the civil war that followed, as many as a thousand ships were either constructed or pressed into service from Greek cities.

Military history

{{Main|Campaign history of the Roman military}} {{further information|Military history of Italy}}

The ruins of the Servian Wall, built during the 4th century BC, one of the earliest ancient Roman defensive walls; by the 3rd century AD it was superseded by the larger Aurelian Walls of Rome

Temple of Janus as seen in the present church of San Nicola in Carcere, in the Forum Holitorium of Rome, Italy, dedicated by Gaius Duilius after his naval victory against the Carthaginians at the Battle of Mylae in 260 BC.

The exact causes and motivations for Rome's military conflicts and expansions during the republic are subject to wide debate. While they can be seen as motivated by outright aggression and imperialism, historians typically take a much more nuanced view. Rome's expansion was driven by short-term defensive goals and other factors including relations with neighboring city-states and kingdoms. In its early history, as Rome successfully defended itself against foreign threats in central and then northern Italy, neighboring city-states sought the protection a Roman alliance would bring. As such, early republican Rome was similar to the Greek hegemonies of the same period. It was an alliance of independent city-states, each with a varying degree of genuine independence engaged in mutual self-protection, but led by Rome. With some important exceptions, successful wars in early republican Rome generally led not to annexation or military occupation, but to the restoration of the way things were. A defeated city would be weakened (sometimes with outright land concessions) and thus less able to resist Romanizing influences, such as Roman settlers seeking land or trade with the growing Roman confederacy. It was also less able to defend itself against its other enemies, making attacks by them more likely, and making a protective alliance with Rome more attractive.

This growing coalition expanded the potential enemies that Rome might face, and moved Rome closer to confrontation with major powers. The result was more alliance-seeking, on the part of both the Roman confederacy and city-states seeking membership (and protection) within that confederacy. While there were exceptions to this (such as military rule of Sicily after the First Punic War), it was not until after the Second Punic War that these alliances started to harden into something more like an empire, at least in certain locations. This shift mainly took place in parts of the west, such as the southern Italian towns that sided with Hannibal.

In contrast, Roman expansion into Spain and Gaul occurred as a mix of alliance-seeking and military occupation. In the 2nd century BC, Roman involvement in the Greek east remained a matter of alliance-seeking, but this time in the face of major powers that could rival Rome. According to Polybius, who sought to trace how Rome came to dominate the Greek east in less than a century, this was mainly a matter of several Greek city-states seeking Roman protection against the Macedonian kingdom and Seleucid Empire in the face of destabilisation created by the weakening of Ptolemaic Egypt. In contrast to the west, the Greek east had been dominated by major empires for centuries, and Roman influence and alliance-seeking led to wars with these empires that further weakened them and therefore created an unstable power vacuum that only Rome could fill. This had some important similarities to (and important differences from) the events in Italy centuries earlier, but this time on a global scale.

Some historians see the growing Roman influence over the east, as with the west, as not a matter of intentional empire-building, but constant crisis management narrowly focused on short-term goals within a highly unstable, unpredictable, and inter-dependent network of alliances and dependencies. However, the question of what motivated Rome's continued imperial expansion up until the establishment of Augustus' Principate remains controversial.

Early Republic (458–274 BC)

Further information: Early Roman army

At first, the wars waged by the Republic were motivated by the desire for expansion and self-defence. They were aimed at protecting Rome itself from neighbouring cities and nations and expanding its territory in the region. One by one, Rome was victorious over its neighbors until it had effectively completed the conquest of the Etruscans and Latins. Rome's successes were interrupted in the early 4th century BC by the Gallic invasion of Italy, led by their chieftain Brennus. At the Battle of Allia River, the Gauls defeated an estimated 15,000 Roman soldiers. Afterwards, the invaders pursued the fleeing army back to Rome, and sacked the city

Rome recovered. And after their victories against the Samnites and rebellious Latin tribes, Rome had complete control of their immediate neighbors. However, Rome had not yet come into conflict with the dominant military powers in the Mediterranean Basin at the time: Carthage and the Greek kingdoms. When they did, Rome was victorious again. At the conclusion of the Pyrrhic War, the Greek king, Pyrrhus's army of 25,000 men was forced to withdraw from Italy and return home, foiling his efforts to assist one of the Greek colonies in Italy. Rome was now confident its armies could contend with the then-dominant military powers, and that the Greek kingdoms were incapable of defending their colonies. Roman forces quickly moved into southern Italy, and subjugated the Greek colonies. With that, the Republic now effectively dominated the Italian peninsula and the reputation of its military gained internationally.

Punic Wars (264–146 BC)

Main article: Punic Wars

Further information: Roman army of the mid-Republic and Rise of Rome

Theatre of the Punic Wars

The three Punic Wars between Rome and Carthage spanned more than 100 years. Between the beginning of the first and the end of the third, both sides suffered enormous casualties, the Carthaginian general Hannibal became a legend, the city of Carthage was razed, and Rome had virtually complete control over the western Mediterranean Sea, Spain, and much of North Africa. While each of the three wars was itself a significant conflict, the Second Punic War featured some of the bloodiest battles in history and is remembered for Hannibal's crossing of the Alps.

The First Punic War began in 264 BC over disputes among the inhabitants of Sicily. The war saw land battles in Sicily early on, but became a predominantly naval conflict, fought in the seas of Sicily and Africa. Before the First Punic War there was no Roman navy to speak of. Fighting the naval power of Carthage forced Rome to quickly build a fleet and train sailors. The first few naval battles were disasters for Rome. However, the expansion of its forces and the deployment of the corvus, which was used to board an enemy ship helped turn the tide. In 241 BC, the Carthaginians sued for peace.

In the years that followed, relations between the two powers remained poor. There were disputes over Rome's annexation of Sardinia and Corsica, reparations, and the return of Roman deserters, and Carthage's relations with the Gauls. Hostilities resumed in 218 BC.

Hannibal and the Second Punic War

A monument to the Battle of Trebbia in Gazzola, Italy

The Second Punic War, also called the "Hadrianic War", has been referred to as the ancient world's first world war. It was fought in Spain, southern France and Northern Italy, as well as Northern Africa by naval forces across the Western Mediterranean. It featured several of the deadliest battles in history. Despite Hannibal's crossing of the Alps and his battlefield victories, the war would be a disaster for Carthage. In the end, Rome would possess virtually all the former Carthaginian territories and leave its enemy in control of only the city of Carthage.

In the years after the end of the first war, Carthaginian Spain was under the control of the Barca family, beginning with Hamilcar, greatly expanded the . Funded by silver, mined from Sierra Morena, Carthaginian Spain, under the Barcas, functioned almost as an independent empire. Hannibal succeeded his father in 228, and by 218 BC, he felt confident enough in his position to raze the Roman protectorate, Saguntum, and the Second Punic War began.

A Roman response arrived in Spain the following year. By then, Hannibal's forces, estimated to be one of the largest assembled in the period, had moved north into Gaul. Under the command of Scipio Calvus, Roman forces successfully engaged Hannibal's brother Hasdrubal, and stymied efforts to reinforce Hannibal after his arrival in Italy. At sea, and in Northern Africa, Rome expanded the conflict, putting additional pressure on Carthage.

Key to Hannibal's strategy were the effort to subvert Rome's allies to his cause, and his taking the war directly to the Roman homeland.

was a product of his education under Greek tutors and the time he spent campaigning with his father.

Continuing distrust led to the renewal of hostilities in the Second Punic War when Hannibal Barca attacked an Iberian town which had diplomatic ties to Rome. Hannibal then crossed the Italian Alps to invade Italy. Hannibal's successes in Italy began immediately, and reached an early climax at the Battle of Cannae, where 70,000 Romans were killed.

A Carthaginian coin possibly depicting Hannibal as Hercules (i.e. Heracles)

The Romans held off Hannibal in three battles, but then Hannibal smashed a succession of Roman consular armies. By this time Hannibal's brother Hasdrubal Barca sought to cross the Alps into Italy and join his brother with a second army. Hasdrubal managed to break through into Italy only to be defeated decisively on the Metaurus River. Unable to defeat Hannibal on Italian soil, the Romans boldly sent an army to Africa under Scipio Africanus to threaten the Carthaginian capital. Hannibal was recalled to Africa, and defeated at the Battle of Zama.

Carthage never recovered militarily after the Second Punic War, but quickly did so economically and the Third Punic War that followed was in reality a simple punitive mission after the neighbouring Numidians allied to Rome robbed/attacked Carthaginian merchants. Treaties had forbidden any war with Roman allies, and defence against robbing/pirates was considered as "war action": Rome decided to annihilate the city of Carthage. Carthage was almost defenceless, and submitted when besieged. However, the Romans demanded complete surrender and removal of the city into the (desert) inland far off any coastal or harbour region, and the Carthaginians refused. The city was besieged, stormed, and completely destroyed.

Ultimately, all of Carthage's North African and Iberian territories were acquired by Rome. Note that "Carthage" was not an 'empire', but a league of Punic colonies (port cities in the western Mediterranean) like the 1st and 2nd Athenian ("Attic") leagues, under leadership of Carthage. Punic Carthage was gone, but the other Punic cities in the western Mediterranean flourished under Roman rule.

Kingdom of Macedonia, the Greek poleis, and Illyria (215–148 BC)

Map showing the southern Balkans and western Asia Minor

Rome's preoccupation with its war with Carthage provided an opportunity for Philip V of the kingdom of Macedonia, located in the north of the Greek peninsula, to attempt to extend his power westward. Philip sent ambassadors to Hannibal's camp in Italy, to negotiate an alliance as common enemies of Rome. However, Rome discovered the agreement when Philip's emissaries were captured by a Roman fleet. The First Macedonian War saw the Romans involved directly in only limited land operations, but they ultimately achieved their objective of pre-occupying Philip and preventing him from aiding Hannibal.

The past century had seen the Greek world dominated by the three primary successor kingdoms of Alexander the Great's empire: Ptolemaic Egypt, Macedonia and the Seleucid Empire. In 202 BC, internal problems led to a weakening of Egypt's position, thereby disrupting the power balance among the successor states. Macedonia and the Seleucid Empire agreed to an alliance to conquer and divide Egypt. Fearing this increasingly unstable situation, several small Greek kingdoms sent delegations to Rome to seek an alliance. The delegation succeeded, even though prior Greek attempts to involve Rome in Greek affairs had been met with Roman apathy. Our primary source about these events, the surviving works of Polybius, do not state Rome's reason for getting involved. Rome gave Philip an ultimatum to cease his campaigns against Rome's new Greek allies. Doubting Rome's strength (a reasonable doubt, given Rome's performance in the First Macedonian War) Philip ignored the request, and Rome sent an army of Romans and Greek allies, beginning the Second Macedonian War. Despite his recent successes against the Greeks and earlier successes against Rome, Philip's army buckled under the pressure from the Roman-Greek army. In 197 BC, the Romans decisively defeated Philip at the Battle of Cynoscephalae, and Philip was forced to give up his recent Greek conquests. The Romans declared the "Peace of the Greeks", believing that Philip's defeat now meant that Greece would be stable. They pulled out of Greece entirely, maintaining minimal contacts with their Greek allies.

With Egypt and Macedonia weakened, the Seleucid Empire made increasingly aggressive and successful attempts to conquer the entire Greek world. Now not only Rome's allies against Philip, but even Philip himself, sought a Roman alliance against the Seleucids. The situation was made worse by the fact that Hannibal was now a chief military advisor to the Seleucid emperor, and the two were believed to be planning an outright conquest not just of Greece, but of Rome itself. The Seleucids were much stronger than the Macedonians had ever been, because they controlled much of the former Persian Empire, and by now had almost entirely reassembled Alexander the Great's former empire.

Roman bronze bust of Scipio Africanus the Elder from the Naples National Archaeological Museum (Inv. No. 5634),
dated mid 1st century BC
Excavated from the Villa of the Papyri at Herculaneum by Karl Jakob Weber, 1750–65.

Fearing the worst, the Romans began a major mobilization, all but pulling out of recently pacified Spain and Gaul. They even established a major garrison in Sicily in case the Seleucids ever got to Italy. This fear was shared by Rome's Greek allies, who had largely ignored Rome in the years after the Second Macedonian War, but now followed Rome again for the first time since that war. A major Roman-Greek force was mobilized under the command of the great hero of the Second Punic War, Scipio Africanus, and set out for Greece, beginning the Roman-Syrian War. After initial fighting that revealed serious Seleucid weaknesses, the Seleucids tried to turn the Roman strength against them at the Battle of Thermopylae (as they believed the 300 Spartans had done centuries earlier). Like the Spartans, the Seleucids lost the battle, and were forced to evacuate Greece. The Romans pursued the Seleucids by crossing the Hellespont, which marked the first time a Roman army had ever entered Asia. The decisive engagement was fought at the Battle of Magnesia, resulting in a complete Roman victory. The Seleucids sued for peace, and Rome forced them to give up their recent Greek conquests. Although they still controlled a great deal of territory, this defeat marked the decline of their empire, as they were to begin facing increasingly aggressive subjects in the east (the Parthians) and the west (the Greeks). Their empire disintegrated into a rump over the course of the next century, when it was eclipsed by Pontus. Following Magnesia, Rome again withdrew from Greece, assuming (or hoping) that the lack of a major Greek power would ensure a stable peace. In fact, it did the opposite.

In 179 BC Philip died. His talented and ambitious son, Perseus, took the throne and showed a renewed interest in conquering Greece. With her Greek allies facing a major new threat, Rome declared war on Macedonia again, starting the Third Macedonian War. Perseus initially had some success against the Romans. However, Rome responded by sending a stronger army. This second consular army decisively defeated the Macedonians at the Battle of Pydna in 168 BC and the Macedonians duly capitulated, ending the war.

Convinced now that the Greeks (and therefore the rest of the region) would not have peace if left alone, Rome decided to establish its first permanent foothold in the Greek world, and divided the Kingdom of Macedonia into four client republics. Yet, Macedonian agitation continued. The Fourth Macedonian War, 150 to 148 BC, was fought against a Macedonian pretender to the throne who was again destabilizing Greece by trying to re-establish the old kingdom. The Romans swiftly defeated the Macedonians at the Second battle of Pydna.

The Achaean League chose this moment to fight Rome but was swiftly defeated. In 146 BC (the same year as the destruction of Carthage), Corinth was besieged and destroyed, which led to the league's surrender. After nearly a century of constant crisis management in Greece, which always led back to internal instability and war when she withdrew, Rome decided to divide Macedonia into two new Roman provinces, Achaea and Macedonia.

Late Republic (147–30 BC)

Further information: Roman army of the late Republic

Jugurthine War (111–104 BC)

The Jugurthine War of 111–104 BC was fought between Rome and Jugurtha of the North African kingdom of Numidia. It constituted the final Roman pacification of Northern Africa, after which Rome largely ceased expansion on the continent after reaching natural barriers of desert and mountain. Following Jugurtha's usurpation of the throne of Numidia, a loyal ally of Rome since the Punic Wars, Rome felt compelled to intervene. Jugurtha impudently bribed the Romans into accepting his usurpation. Jugurtha was finally captured not in battle but by treachery.

Celtic threat (121 BC) and Germanic threat (113–101 BC)

In 121 BC, Rome came into contact with two Celtic tribes (from a region in modern France), both of which they defeated with apparent ease. The Cimbrian War (113–101 BC) was a far more serious affair than the earlier clashes of 121 BC. The Germanic tribes of the Cimbri and the Teutons migrated from northern Europe into Rome's northern territories, and clashed with Rome and her allies. At the Battle of Aquae Sextiae and the Battle of Vercellae both tribes were virtually annihilated, which ended the threat.

Internal unrest (135–71 BC)

A Roman naval bireme depicted in a relief from the Temple of Fortuna Primigenia in Praeneste (Palastrina), which was built c. 120 BC; exhibited in the Pius-Clementine Museum (Museo Pio-Clementino) in the Vatican Museums.

The extensive campaigning abroad by Roman generals, and the rewarding of soldiers with plunder on these campaigns, led to a general trend of soldiers becoming increasingly loyal to their generals rather than to the state. Rome was also plagued by several slave uprisings during this period, in part because vast tracts of land had been given over to slave farming in which the slaves greatly outnumbered their Roman masters. In the 1st century BC at least twelve civil wars and rebellions occurred. This pattern continued until 27 BC, when Octavian (later Augustus) successfully challenged the Senate's authority, and was made princeps (first citizen).

Between 135 BC and 71 BC there were three "Servile Wars" involving slave uprisings against the Roman state. The third and final uprising was the most serious, involving ultimately between 120,000 and 150,000 slaves under the command of the gladiator Spartacus. In 91 BC the Social War broke out between Rome and its former allies in Italy when the allies complained that they shared the risk of Rome's military campaigns, but not its rewards. Although they lost militarily, the allies achieved their objectives with legal proclamations which granted citizenship to more than 500,000 Italians.

The internal unrest reached its most serious state, however, in the two civil wars that were caused by the clash between generals Gaius Marius and Lucius Cornelius Sulla starting from 88 BC. In the Battle of the Colline Gate at the very door of the city of Rome, a Roman army under Sulla bested an army of the Marius supporters and entered the city. Sulla's actions marked a watershed in the willingness of Roman troops to wage war against one another that was to pave the way for the wars which ultimately overthrew the Republic, and caused the founding of the Roman Empire.

Conflicts with Mithridates (89–63 BC) and the Cilician pirates (67 BC)

Mithridates the Great was the ruler of Pontus, a large kingdom in Asia Minor (modern Turkey), from 120 to 63 BC. Mithridates antagonised Rome by seeking to expand his kingdom, and Rome for its part seemed equally eager for war and the spoils and prestige that it might bring. In 88 BC, Mithridates ordered the killing of a majority of the 80,000 Romans living in his kingdom. The massacre was the official reason given for the commencement of hostilities in the First Mithridatic War. The Roman general Lucius Cornelius Sulla forced Mithridates out of Greece proper, but then had to return to Italy to answer the internal threat posed by his rival, Gaius Marius. A peace was made between Rome and Pontus, but this proved only a temporary lull.

The Second Mithridatic War began when Rome tried to annex a province that Mithridates claimed as his own. In the Third Mithridatic War, first Lucius Licinius Lucullus and then Pompey the Great were sent against Mithridates and his Armenian ally Tigranes the Great. Mithridates was finally defeated by Pompey in the night-time Battle of the Lycus.

The Mediterranean had at this time fallen into the hands of pirates, largely from Cilicia. The pirates not only strangled shipping lanes but also plundered many cities on the coasts of Greece and Asia. Pompey was nominated as commander of a special naval task force to campaign against the pirates. It took Pompey just forty days to clear the western portion of the sea of pirates and restore communication between Iberia (Spain), Africa, and Italy.

Caesar's early campaigns (59–50 BC)

Map of the Gallic Wars

During his term as praetor in the Iberian Peninsula (modern Portugal and Spain), Pompey's contemporary Julius Caesar defeated two local tribes in battle. After his term as consul in 59 BC, he was appointed to a five-year term as the proconsular Governor of Cisalpine Gaul (part of current northern Italy), Transalpine Gaul (current southern France) and Illyria (part of the modern Balkans). Not content with an idle governorship, Caesar strove to find reason to invade Gaul (modern France and Belgium), which would give him the dramatic military success he sought. When two local tribes began to migrate on a route that would take them near (not into) the Roman province of Transalpine Gaul, Caesar had the barely sufficient excuse he needed for his Gallic Wars, fought between 58 BC and 49 BC.

Caesar defeated large armies at major battles 58 and 57 BC. In 55 and 54 BC he made two expeditions into Britain, the first Roman to do so. Caesar then defeated a union of Gauls at the Battle of Alesia, completing the Roman conquest of Transalpine Gaul. By 50 BC, all of Gaul lay in Roman hands. Gaul never regained its Celtic identity, never attempted another rebellion, and, except for the Crisis of the Third Century, remained loyal to Rome until the fall of the empire in 476.

Triumvirates and Caesarian ascension (53–30 BC)

Further information: Death of Cleopatra

By 59 BC an unofficial political alliance known as the First Triumvirate was formed between Gaius Julius Caesar, Marcus Licinius Crassus, and Gnaeus Pompeius Magnus ("Pompey the Great") to share power and influence. In 53 BC, Crassus launched a Roman invasion of the Parthian Empire (modern Iraq and Iran). After initial successes, he marched his army deep into the desert; but here his army was cut off deep in enemy territory, surrounded and slaughtered at the Battle of Carrhae in which Crassus himself perished. The death of Crassus removed some of the balance in the Triumvirate and, consequently, Caesar and Pompey began to move apart. While Caesar was fighting in Gaul, Pompey proceeded with a legislative agenda for Rome that revealed that he was at best ambivalent towards Caesar and perhaps now covertly allied with Caesar's political enemies. In 51 BC, some Roman senators demanded that Caesar not be permitted to stand for consul unless he turned over control of his armies to the state, which would have left Caesar defenceless before his enemies. Caesar chose civil war over laying down his command and facing trial.

By the spring of 49 BC, the hardened legions of Caesar crossed the river Rubicon, the legal boundary of Roman Italy beyond which no commander might bring his army, and swept down the Italian peninsula towards Rome, while Pompey ordered the abandonment of Rome. Afterwards Caesar turned his attention to the Pompeian stronghold of Hispania (modern Spain) but decided to tackle Pompey himself in Greece. Pompey initially defeated Caesar, but failed to follow up on the victory, and was decisively defeated at the Battle of Pharsalus in 48 BC, despite outnumbering Caesar's forces two to one, albeit with inferior quality troops. Pompey fled again, this time to Egypt, where he was murdered.

Detail from the Ahenobarbus relief showing (centre-right) two Roman foot-soldiers ca. 122 BC. Note the Montefortino-style helmets with horsehair plume, chain mail cuirasses with shoulder reinforcement, oval shields with calfskin covers, gladius and pilum

This mid-1st-century-BC Roman wall painting in Pompeii, Italy, showing Venus holding a cupid is most likely a depiction of Cleopatra VII of Ptolemaic Egypt as Venus Genetrix, with her son Caesarion as the cupid, similar in appearance to the now lost statue of Cleopatra erected by Julius Caesar in the Temple of Venus Genetrix (within the Forum of Caesar). The owner of the House of Marcus Fabius Rufus at Pompeii walled off the room with this painting, most likely in immediate reaction to the execution of Caesarion on orders of Octavian in 30 BC, when artistic depictions of Caesarion would have been considered a sensitive issue for the ruling regime.

Pompey's death did not end the civil war, as Caesar's many enemies fought on. In 46 BC Caesar lost perhaps as much as a third of his army, but ultimately came back to defeat the Pompeian army of Metellus Scipio in the Battle of Thapsus, after which the Pompeians retreated yet again to Hispania. Caesar then defeated the combined Pompeian forces at the Battle of Munda.

Caesar was now the primary figure of the Roman state, enforcing and entrenching his powers. His enemies feared that he had ambitions to become an autocratic ruler. Arguing that the Roman Republic was in danger, a group of senators hatched a conspiracy and assassinated Caesar at a meeting of the Senate in March 44 BC. Mark Antony, Caesar's lieutenant, condemned Caesar's assassination, and war broke out between the two factions. Antony was denounced as a public enemy, and Caesar's adopted son and chosen heir, Gaius Octavianus, was entrusted with the command of the war against him. At the Battle of Mutina Mark Antony was defeated by the consuls Hirtius and Pansa, who were both killed.

Octavian came to terms with Caesarians Antony and Marcus Aemilius Lepidus in 43 BC when the Second Triumvirate was formed. In 42 BC Mark Antony and Octavian fought the Battle of Philippi against Caesar's assassins Brutus and Cassius. Although Brutus defeated Octavian, Antony defeated Cassius, who committed suicide. Brutus did likewise soon afterwards.

However, civil war flared again when the Second Triumvirate of Octavian, Lepidus and Mark Antony failed. The ambitious Octavian built a power base of patronage and then launched a campaign against Mark Antony. At the naval Battle of Actium in 31 BC off the coast of Greece, Octavian decisively defeated Antony and Cleopatra of Ptolemaic Egypt. Octavian was granted a series of special powers including sole "imperium" within the city of Rome, permanent consular powers and credit for every Roman military victory, since all future generals were assumed to be acting under his command. In 27 BC Octavian was granted the use of the names "Augustus", indicating his primary status above all other Romans, "Princeps", which he used to refer to himself as in public, and he adopted the title "Imperator Caesar" making him the first Roman Emperor.