The Roman army of the late Republic refers to the armed forces deployed by the late Roman Republic, from the beginning of the first century BC until the establishment of the Imperial Roman army by Augustus in 30 BC.
Shaped by major social, political, and economic change, the late Republic saw the transition from the Roman army of the mid-Republic, which was a temporary levy based solely on the conscription of Roman citizens, to the Imperial Roman army of the Principate, which was a standing, professional army based on the recruitment of volunteers.
Continuous expansion, wars, conflicts, and the acquisition of a growing, overseas territory led to an increasing degree of professionalism within the army. The late-Republic saw much of its action taking place within the Roman borders and between Roman commanders as they vied for control of the Republic. There was a significant intertwining of military and politics in the acquisition and maintenance of power. After the Social War, and following the establishment of the First Triumvirate by Julius Caesar, Licinius Crassus, and Pompeius Magnus, there grew an emphasis on the expansion of a united Republic toward regions such as Britain and Parthia. The effort to quell the invasions and revolts of non-Romans persisted throughout the period, from Marius’ battles with the wandering Germans in Italy to Caesar's campaign in Gaul.
After the completion of the Social War in 88 BC, Roman citizenship was granted to all its Italian allies (the socii) south of the Po River. The alae were abolished, and the socii were from now on recruited directly into uniformly organized and equipped legions. The non-Italian allies that had long fought for Rome (e.g., Gallic and Numidian cavalry) continued to serve alongside the legions but remained irregular units under their own leaders.
For reasons that remain uncertain to this day, the structure of the Roman army changed dramatically during the late Republic. The maniple, which had been the standard unit throughout the mid-Republic, was replaced by the cohort as the new standard tactical unit of the legions, while the Roman citizen cavalry (equites) and light infantry (velites) disappeared from the battlefield. Traditionally, many of these changes have been attributed to the reforms of Gaius Marius (see Marian reforms), but some scholars argue that they may have happened far more gradually.
Main sources The main sources for the army's organization and practices in this period are the publications De Bello Gallico and De Bello Civilli, begun by the Roman general Julius Caesar and finished by his subordinates.
Conflicts and expansion During the late Republic, Rome was in a state of almost continuous warfare and civil war. Ambitious commanders, driven by a desire to distinguish themselves from their contemporaries, led massive campaigns to expand the empire's borders far beyond the region of Italy. In the “most intensive period of conquest in Rome's history,” Rome dramatically increased in size by adding large territories in Gaul, North Africa, Asia Minor, Cyprus, Crete, and the Levant to its territory. The frequency of war, the prolonged duration of the campaigns, and the growing demand for garrisons resulted in the legions to develop a much more permanent and professional character. As the length of military service increased, the legionaries began to view the army not just as an interruption of normal life, but as a career in itself.
Campaign history: military in politics
Jugurthine War After receiving permission from the Plebeian Council to command the army in 107 BC, Gaius Marius marched through Numidia to take the town of Capsa whose entire population was either killed or sold into slavery. This action was later criticized as “against the law of war” by Roman historian Sallust. Marius then swiftly assured victory after marching 600 miles to capture the Jugurthine Royal Treasury. The campaign displayed the effectiveness of the proletarian army in battle.
Conflict with wandering Germanic tribes In 105 BC, two Roman armies were defeated by the Cimbri and Teutones in the Battle of Arausio. These forces were led by Servilius Caepio of a firmly ingrained Roman heritage and Mallius Maximus from a newer, aspiring family. The defeat is credited to a lack of coordination due to strife between them as Caepio refused to work alongside someone not of noble blood.
The constant fear of Germanic invasion allowed the Populist faction a foothold to have Gaius Marius reelected in 104, 103, and 102 BC. Marius showcased his army's capability once again, first massacring 90,000 of the 100,000 Teutones soldiers, which included women and children, implementing a well-coordinated rear ambush tactic. He subsequently overpowered the Cimbri at Vercellae in what is now Vercelli, Italy. Following these battles, Marius eradicated the equites and thus the light cavalry which doubled as a political attack against the aristocratic class.
Since proletarians had no land to return to, Marius, now beginning his 6th consulship, joined his men in the Forum to battle the Senate who refused to provide them with land. This exemplifies the growing trend of applying military force to obtain a political resolution.
Social War The Social War opened in 91 BC when Italians began to revolt because the Senate would not grant them Roman citizenship even though they showed loyalty in fighting for Rome in the past. The revolt was headed by the Marsi and Samnites who established a capital, senate, and two commanders at Corfinium which was renamed Italia. Some Italians remained on Rome's side including the Etruscans, Umbrians, Campanians, Greeks, and Latins. The Roman force consisted of 150,000 men against 100,000, but the rebellion was extinguished when, in 89 BC, Rome presented a citizenship offer to all Italians that surrendered.
Civil Wars At the completion of the Social War in 89 BC, Lucius Cornelius Sulla Felix marched on Rome with six legions, who devoted their loyalty solely to him, as a means of coercing the Plebeian Council to grant him authority to fight King Mithridates of Pontus who invaded the Roman province of Asia. This sparked factional fighting and the murders of important Romans such as Quintus Pompeius Rufus. Lucius Cornelius Cinna and Gaius Marius teamed up to use their armies to sack and loot Rome and declare themselves co-consuls after starving it out. It would be Marius’ seventh and final term. In 83 BC, following his capture of Athens from Mithridates, Sulla returned to Rome, joined his army of 35,000 veterans with three legions raised by the young Pompey to defeat a lone consul's 100,000 newly recruited.
The First Triumvirate in 60 BC consisted of Pompey, Marcus Licinius Crassus, and Julius Caesar; the latter was granted command in Cisalpine Gaul, Illyricum, and Transalpine Gaul as a result of some recent successes: commanding at a victorious siege of Mytilene in 80 BC to receive a civic crown, defeating pirates in naval warfare, commanding in the Pontic War of 74, and subduing Spanish tribes in 61. Caesar called upon his forces to threaten the Senate into providing land for Pompey's veterans.
Annexations achieved by Caesar (although he cunningly called them alliances) include lands of many Gallic tribes such as the Aedui, Belgic tribes like the Nervii, and Germanic tribes including the Usipetes and Tencteri. Even though he characteristically drove his men to their limits in these battles, those legions would grow intensely loyal to him which would become very important in the near future.
As Caesar's legal command over his 13 legions was running out, he famously crossed the Rubicon River with just one legion, purportedly stating alea iacta est ("the die is cast"). He defeated the larger Pompeian armies through the experience of his men and clever use of strategy, even employing the pilum as a bayonet to combat Pompey's 7–1 cavalry advantage.
Caesar's assassination led to the creation of the Second Triumvirate of Octavian (to become known as Augustus), Mark Antony, and Marcus Aemilius Lepidus in 43 BC. Exemplifying the mobility of loyalty in the army of this period, Octavian was able to raise legions without legal command as a result of his Caesarian connection, two of which had defected from Antony. The Triumvirate was short lived and inner strife led to further civil war. Thanks to the military prowess of his general Agrippa, Octavian was able to gain control of the West's entire army of 45 legions and navy of 500–600 ships. Even though the Roman military power concentrated on heavy infantry, the importance of naval warfare was displayed in the final Battle of Actium, where Octavian achieved total victory. He then acted to consolidate the military in preparation to transition the Roman government to the Principate.
Background: The Polybian army The organization of the army of the mid-republic (also known as the "Polybian" or Manipular army) was described by the Greek historian Polybius in the mid-second century BC The basic military unit of the Polybian army was the maniple, which numbered about 120 men, and was subdivided into two centuries of 60 men. The standard legion contained thirty maniples organized into three distinct lines, and consisted of about 4,200 infantrymen and 300 Roman citizen cavalrymen (equites). The actual size of the legions, however, often depended on the particular situation, and sources mention that in times of need legions could number between 5,000 and 6,000 men.
Based on their property and age, the infantrymen were divided into four specific groups, and were organized and equipped according to that particular group. The first group, which formed the first line of the Polybian legion, was that of the hastati. These men were recruited from the younger men eligible for service and were probably in their late teens or early 20s. The second line was formed by the principes, who were drawn from men in their later 20s or early 30s, and the last line was made up by the triarii, who were the oldest and (supposedly) most experienced men in the army. While there were normally 1,200 hastati and 1,200 principes in every legion, the triarii numbered just 600. The poorest and youngest of the citizens fought in the legions as light infantry (velites), of which there were usually 1,200 in each legion.
Beside the troops levied from the eligible Roman population served the troops provided by Rome's allies. These troops, primarily recruited from Rome's allies on the Italian peninsula (or the socii), were organized in so-called alae (ala singular) or 'wing' (referring to their location on the flanks of the Roman legions).
Marian reforms
Army structure and organization
Legionary infantry
By the first decades of the 1st century, the cohort had replaced the maniple as the standard tactical unit of the legions. The three lines of the manipular legion were combined to form the cohort, which generally numbered about 480 to 500 men. Maniples and centuriae continued to be used to as military and administrative subdivisions for the cohort. There were six centuriae in a cohort, which were now all 80-men strong. The legion was now composed of ten cohorts rather than thirty maniples, and numbered an average of about 5,000 men. The legionaries no longer needed to provide their own equipment and were now all equipped and organized as heavy infantry with pilum and gladius.