The Army

The early Roman army (c. 500 BCE) 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 BCE, 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 BCE, 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 BCE, 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 BCE, 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 BCE, 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 CE) 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 CE, 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.

The Navy

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 BCE 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 CE), 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.