By the size of the Roman army is meant the changes (increases and reductions) in the number of its contingents: legions, auxiliaries, Praetorian cohorts, Urban cohorts, vigiles, and naval forces over the course of twelve centuries – from 753 BC to AD 476 (the Fall of the Western Roman Empire).
Regal period (753 – 509 BC)
After the founding of Rome, legend has it that the first king, Romulus established the original Roman legion with 3,000 soldiers and 300 cavalry,Plutarch, Life of Romulus, 13.1. which might have been doubled when the city of Rome was expanded by union with the Sabines, coming to a total of 6,000 infantry and 600 cavalry.Plutarch, Life of Romulus, 20.1. By the time of Servius Tullius or perhaps the Tarquini the forces had increased once more, bringing the number of infantry to 17,000 and of cavalry to 1,800.Livy, Ab Urbe condita libri, 1.43.P.Connolly, Greece and Rome at war, London 1998, p.95.
Republican period (509 – 31 BC)
We know from Livy that at the time of the Latin War (340–338 BC) there were normally two armies enlisted, composed of four legions of 4,200–5,000 infantry and 300 cavalry each, for a total armed force of 16,800–20,000 infantry and 1,200 cavalry.Livy, Ab Urbe condita libri, 8.8.14. with an equal number of allied infantry and three times as many cavalry – i.e. 16,800–20,000 infantry and 3,600 cavalry (called socii).Livy, Ab Urbe condita libri, 8.8.15.
At the time of the Pyrrhic War, the Roman army in the field consisted of four armies,Livy, Periochae of Ab Urbe condita libri, 9.30. each of which contained two legions of Roman citizens and two units of allies. Each legion consisted of 4,200–5,000 infantryPolybius, Histories, 6.20.8–9. and 300 cavalry, while the allied units had an equal number of infantry but three times as many cavalry (900 cavalry per unit).Polybius, Histories, 6.26.7. The complete total, therefore, sums to around 80,000 infantry and 10,000 cavalry.P.Connolly, L'esercito romano, Mondadori, Milano 1976, p.10-11.
Around the middle of the third century BC, the Roman army consisted of an occupation force in Sicily and Tarentum (two legions of 4,200 infantry and 300 cavalry each), two consulary armies (both composed of two reinforced legions of around 5,200 infantry and 300 cavalry each) and about 30,000 allied soldiers (including 2,000 cavalry) in permanent active service, with another 90,000 in reserve, ready to intervene if necessary and made up of 50,000 Etruscans and Sabines (including 4,000 cavalry), 20,000 Umbrians and Sarsinans, 20,000 Veneti and Cenomani. The complete armed forces, therefore, could have reached 150,000 individuals, of which only 30,000 were Romans (6 legions).
During the war against Hannibal, the Roman army came to number 23 legionsGiovanni Brizzi, Scipione e Annibale. La guerra per salvare Roma, p.97. including Roman citizens and Socii (in 212–211 BC), deployed in Italia, Illyricum, Sicily, Sardinia, Cisalpine Gaul and on the borders of Macedon. These numbered around 115,000 infantry and 13,000 cavalry (extrapolating from the numbers provided by Polybius), plus the small squadron of the Scipiones in Hispania, the fleets in the Ionian Sea (50 ships) and around Sicily (100 ships).
At the death of Julius Caesar there were 37 legions in the whole Roman world,L.Keppie, The making of the roman army, from Republic to Empire, Oklahoma 1998, p.201. including 6 in Macedonia, 3 in Africa and 10 in the eastern provinces.Keppie, The making of the roman army, from Republic to Empire, 1998, p.113. We must not forget that these troops were extremely mobile, that their hiberna (winter quarters) were still rudimentary and little more than a "marching camp".
At the end of civil war between Mark Antony and Octavian there were around 60 legions, although few were at full strength.
Imperial period
High empire (31 BC – AD 284) In the time of Tiberius, according to a study by Le Bohec, the legionary forces may have reached the considerable figure of 125,000 men, divided into 25 legions, and with additional auxiliary forces for a total of 250,000 soldiers (including around 30,000 cavalry). In addition to this there were 10,000 soldiers garrisoned in the capital, including the Praetorian guard, the urban cohorts, vigiles, equites singulares Augusti and 40,000 marines in the navy.Y.Le Bohec, L'esercito romano d Augusto alla fine del III secolo, Roma 2008, pp.34–36.
Under Trajan the number of legions was brought to 30 (around 165,000 legionaries), while the number of auxiliaries was increased to 380 units (for a total of around 200,000–220,000 men, including 70,000 cavalry), totalling 360,000–380,000 soldiers.
Holder's 2003 study broadly affirms these figures, finding that the Roman army contained about 380,000 soldiers: 154,000 legionaries and 223,000 auxiliaries (excluding the forces in Rome, the fleets of Ravenna, Misenum, and smaller naval deployments on the Rhine, Danube, Mediterranean coast, Black Sea coast and English Channel):
+Roman Army: summary of the distribution by province, around 135 ADHolder, (2003) p.145.Province Modernequivalent Alae(n.miliarie) Cohortes(n.miliarie) Totalunits X Cavalry* Infantry Totalauxiliaries X Num.legions Legionaryinfantry X GrandtotalBritanniaEngland and Wales11 (1)45 (6)5610.68825.52036.208316.50052.708Rhine frontierGermania InferiorLow Countries617234.5128.16012.672211.00023.672Germania SuperiorAlsace322 (1)253.33610.88014.216211.00025.216Danube frontierRhaetia/NoricumSouth GermanySwitzerland/Austria7 (1)20 (5)275.28011.22016.50015.50022.000PannoniaHungary west. Slovenia11 (2)21 (4)328.30411.36019.664316.50036.164Moesia Superiornorthern Serbia210121.8644.8006.664211.00017.664Moesia InferiorBulgaria512173.5205.7609.28015.50014.780Three DaciaeRomania11 (1)32 (8)437.32817.92025.248211.00036.248Eastern frontierCappadociaTurkey415 (2)193.3687.84011.208316.50027.708Syria, Judaea,and ArabiaSyria, Palestine,Jordan and Israel12 (1)43 (3)5510.24021.60031.840527.50059.340North AfricaEgyptEgypt411153.0085.2808.288211.00019.288Mauretania and AfricaTunisia, Algeriaand Morocco10 (1)30 (1)407.79614.72022.51615.50028.016Internal provinces215172.2247.2009.42415.50014.924Great grandtotal88 (7)293 (30)38171.468152.260223.72828154.000377.728
In the time of Emperor Septimius Severus, the Roman army reached around 500,000 total individuals, with 33 legions (182,000 legionaries) and more than 400 auxiliary units (around 250,000 auxiliariesJ.C.Spaul, Ala, (1996), 257–60; Cohors 2, (2000) 523–7; identifies 4 alae and 20–30 cohortes enlisted in the late second and early third centuries. of which around 75,000 served as cavalry). In the course of the Crisis of the third century, the legions reached 36 units – equivalent to 200,000 soldiers – so that more than 500,000 individuals were under arms.
Late Roman empire (284–337)
With the institution of the Tetrarchy by Diocletian, the total number of legions reached 53–56 in AD 300.J.R.González, Historia de las legiones romanas, p. 709-710. The garrison of Rome underwent an important expansion (which may already have occurred during the third century). There were 10 Praetorian cohorts of 1,000 men each and 1,000 equites singulares, for a total of 24,000 men.Y.Le Bohec, Armi e guerrieri di Roma antica. Da Diocleziano alla caduta dell'impero, Roma 2008, pp.27–28. The fleet in this period is meant to have reached around 45,500 men, according to John Lydus, a bureaucrat who wrote in the time of Justinian.
The accession of Constantine to the throne and the revival of the dynastic monarchy saw the number of Roman legions increased for a final time to 64–67 by the time of his death in 337.J.R.González, Historia de las legiones romanas, p. 711-712. The figure might be further augmented if the forces of Barbarian foederati are included as part of the Roman army.
Thus, scholarly estimates of the number of soldiers of the first three centuries of the Roman empire can be summarised as follows: + Roman army 284–337 Military units Tiberius24 Trajan107 Hadrianca. 135 Marcus Aurelius166/7 Septimius Severus211 Aurelian275 Diocletian305 Constantine I337Legionaries125,00025 legions of 5,000 men each165,00028 legions of 5,500 each (the first cohort had double normal force from the time of the Emperor Domitian (r. 81–96)154,00030 legions of 5,500 men each165,000182,000Goldsworthy (2000) p.152 (map): 33 legions of 5,500 each209,000J.R.González, Historia de las legiones romanas, p. 708-709: 37–38 legions of 5,500 each in 275.265/280,000Assuming there were still 5,000 soldiers per legion.320/335,000Legions2530J.R.González, Historia de las legiones romanas, p. 701-702: 30 legions of 5,500.28Holder, (2003) p.12030J.R.González, Historia de las legiones romanas, p. 703-704: 30 legions of 5,500.3337–3853–5664–67Auxiliaries125,000Tacitus, Annales, 4.5.224.000224.000224.000D.Kennedy, L'Oriente, in Il mondo di Roma imperiale. La formazione, edited by J.Wacher, Bari 1989, p.299, fig.11.1.250.000~250.000 (?)~250.000 (?)~243/228.000 (?)Praetorian guard andother garrisons of Rome~10,000Goldsworthy (2003) p.58, 9 cohorts of 480 men each as well as a Germanic guard, urban cohorts and vigiles.~15,000Goldsworthy (2003) p.58, 9 cohorts of 800 men each as well as 2,000 equites singulares, urban cohorts, and vigiles.~15,000~15,000~20,000Under Septimius Severus, the Praetorian cohorts were each increased to 1,000 men.~20,00024,00018,000The Praetorian guard was abolished by Constantine in 312.Roman navy~40,000~50,000The increase was caused by the expansion of the fleets along the Danube, Black Sea and English Channel after the Roman conquest of Britain.~50,000~50,000~50,000~45,000Y.Le Bohec, Armi e guerrieri di Roma antica. Da Diocleziano alla caduta dell'impero, Roma 2008, pp.41–42.45.50064.000Treadgold (1997), p. 145Total Roman forces300,000454,000443,000Hassall (2000) p.320 estimate around 380,000, excluding the fleet.454,000502,000MacMullen "How Big was the Roman Army?" in KLIO (1979) p.454 estimates around 438,000, excluding the fleet.524,000584,000–599,500Agathias (On the reign of Justinian, 5.13) as interpreted by A. H. M. Jones The Later Roman Empire, 284–602: A Social, Economic and Administrative Survey, Baltimore 1964, vol.II, pp. 679–686, claims 645,000 soldiers.645,000
Thanks especially to the Notitia dignitatum (datable to the beginning of the fifth century), some modern authors, such as A. H. M. Jones have proposed a reconstruction of the dimensions of the Roman army in the time of Constantine, using the list of military units present in the eastern and western portions of the empire. Jones's proposal could be summarised thus: 600,000 soldiers total, including 104,000 comitatenses in the west, 113,000 comitatenses in the east, 135,000 limitanei in the west, and 248,000 in the east.Carrié, Eserciti e strategie, in vol.XVIII della Storia Einaudi dei Greci e dei Romani, Milano-Torino 2008, pp.135–136.A.H.M.Jones, The later roman empire (284–602), vol.II, App.II, Tav.XV, Oklahoma 1986, pp. 1.449–1.450. This estimate was said to be affirmed by Agathias (On the reign of Justinian, 5.13) who wrote at the time of Justinian that in "earlier times" (which A. H. M. Jones suggests was before 395, most probably in the time of Diocletian and Constantine,The Later Roman Empire, 284–602: A Social, Economic and Administrative Survey, Baltimore 1964, vol.I, pp. 679–686), the army had numbered 645,000 soldiers.