Early Roman army (c. 500 BC to c. 300 BC)
The Early Roman army was the armed force of the Roman Kingdom and of the early Republic During this period, when warfare chiefly consisted of small-scale plundering raids, it has been suggested that the army followed Etruscan or Greek models of organisation and equipment. The early Roman army was based on an annual levy.
The infantry ranks were filled with the lower classes while the cavalry (equites or celeres) were left to the patricians, because the wealthier could afford horses. Moreover, the commanding authority during the regal period was the high king. Until the establishment of the Republic and the office of consul, the king assumed the role of commander-in-chief. However, from about 508 BC Rome no longer had a king. The commanding position of the army was given to the consuls, "who were charged both singly and jointly to take care to preserve the Republic from danger".
The term legion is derived from the Latin word legio; which ultimately means draft or levy. At first there were only four legions. These legions were numbered "I" to "IIII", with the fourth being written as such and not "IV". The first legion was seen as the most prestigious. The bulk of the army was made up of citizens. These citizens could not choose the legion to which they were allocated. Any man "from ages 16–46 were selected by ballot" and assigned to a legion.
Until the Roman military disaster of 390 BC at the Battle of the Allia, Rome's army was organised similarly to the Greek phalanx. This was due to Greek influence in Italy "by way of their colonies". Patricia Southern quotes ancient historians Livy and Dionysius in saying that the "phalanx consisted of 3,000 infantry and 300 cavalry". Each man had to provide his equipment in battle; the military equipment which he could afford determined which position he took in the battle. Politically they shared the same ranking system in the Comitia Centuriata; which ultimately vis-à-vis placed the men on the battlefield.
Roman army of the mid-Republic (c. 300–88 BC)
The Roman army of the mid-Republic was also known as the "manipular army" or the "Polybian army" after the Greek historian Polybius, who provides the most detailed extant description of this phase. The Roman army started to have a full-time strength of 150,000 at all times and 3/4 of the rest were levied.
During this period, the Romans, while maintaining the levy system, adopted the Samnite manipular organisation for their legions and also bound all the other peninsular Italian states into a permanent military alliance (see Socii). The latter were required to supply (collectively) roughly the same number of troops to joint forces as the Romans to serve under Roman command. Legions in this phase were always accompanied on campaign by the same number of allied alae (Roman non-citizen auxiliaries), units of roughly the same size as legions.
After the 2nd Punic War (218–201 BC), the Romans acquired an overseas empire, which necessitated standing forces to fight lengthy wars of conquest and to garrison the newly gained provinces. Thus the army's character mutated from a temporary force based entirely on short-term conscription to a standing army in which the conscripts were supplemented by a large number of volunteers willing to serve for much longer than the legal six-year limit. These volunteers were mainly from the poorest social class, who did not have plots to tend at home and were attracted by the modest military pay and the prospect of a share of war booty. The minimum property requirement for service in the legions, which had been suspended during the 2nd Punic War, was effectively ignored from 201 BC onward in order to recruit sufficient volunteers. Between 150-100 BC, the manipular structure was gradually phased out, and the much larger cohort became the main tactical unit. In addition, from the 2nd Punic War onward, Roman armies were always accompanied by units of non-Italian mercenaries, such as Numidian light cavalry, Cretan archers, and Balearic slingers, who provided specialist functions that Roman armies had previously lacked.
Roman army of the late Republic (88–30 BC)
The Roman army of the late Republic (88–30 BC) marks the continued transition between the conscription-based citizen-levy of the mid-Republic and the mainly volunteer, professional standing forces of the imperial era. The main literary sources for the army's organisation and tactics in this phase are the works of Julius Caesar, the most notable of a series of warlords who contested for power in this period. As a result of the Social War (91–88 BC), all Italians were granted Roman citizenship, the old allied alae were abolished and their members integrated into the legions. Regular annual conscription remained in force and continued to provide the core of legionary recruitment, but an ever-increasing proportion of recruits were volunteers, who signed up for 16-year terms as opposed to the maximum 6 years for conscripts. The loss of ala cavalry reduced Roman/Italian cavalry by 75%, and legions became dependent on allied native horse for cavalry cover. This period saw the large-scale expansion of native forces employed to complement the legions, made up of numeri ("units") recruited from tribes within Rome's overseas empire and neighbouring allied tribes. Large numbers of heavy infantry and cavalry were recruited in Spain, Gaul and Thrace, and archers in Thrace, Anatolia and Syria. However, these native units were not integrated with the legions, but retained their own traditional leadership, organisation, armour and weapons.
Imperial Roman army (30 BC – AD 284)
During this period the Republican system of citizen-conscription was replaced by a standing professional army of mainly volunteers serving standard 20-year terms (plus 5 as reservists), although many in the service of the empire would serve as many as 30 to 40 years on active duty, as established by the first Roman emperor, Augustus (sole ruler 30 BC – AD 14). Regular annual conscription of citizens was abandoned and only decreed in emergencies (e.g. during the Illyrian revolt 6–9 AD). Under Augustus there were 28 legions, consisting almost entirely of heavy infantry, with about 5,000 men each (total 125,000). This had increased to a peak of 33 legions of about 5,500 men each (c. 180,000 men in total) by AD 200 under Septimius Severus. Legions continued to recruit Roman citizens, mainly the inhabitants of Italy and Roman colonies, until 212. Legions were flanked by the auxilia, a corps of regular troops recruited mainly from peregrini, imperial subjects who did not hold Roman citizenship (the great majority of the empire's inhabitants until 212, when all were granted citizenship). Auxiliaries, who served a minimum term of 25 years, were also mainly volunteers, but regular conscription of peregrini was employed for most of the 1st century AD. The auxilia consisted, under Augustus, of about 250 regiments of roughly cohort size, that is, about 500 men (in total 125,000 men, or 50% of total army effectives). Under Severus the number of regiments increased to about 400, of which about 13% were double-strength (250,000 men, or 60% of total army). Auxilia contained heavy infantry equipped similarly to legionaries, and almost all the army's cavalry (both armoured and light), and archers and slingers.
Later Roman army (284–476 AD) continuing as East Roman army (476–641 AD)
depicting the liberation of a besieged city by a relief force, with those defending the walls making a sortie
(i.e. a sudden attack against a besieging enemy from within the besieged town); Western Roman Empire
, early 5th Century AD
The Late Roman army period stretches from (284–476 AD and its continuation, in the surviving eastern half of the empire, as the East Roman army to 641). In this phase, crystallised by the reforms of the emperor Diocletian (ruled 284–305 AD), the Roman army returned to regular annual conscription of citizens, while admitting large numbers of non-citizen barbarian volunteers. However, soldiers remained 25-year professionals and did not return to the short-term levies of the Republic. The old dual organisation of legions and auxilia was abandoned, with citizens and non-citizens now serving in the same units. The old legions were broken up into cohorts or even smaller units. At the same time, a substantial proportion of the army's effectives were stationed in the interior of the empire, in the form of comitatus praesentales, armies that escorted the emperors.
Middle Byzantine army (641–1081 AD)
The Middle Byzantine army (641–1081 AD) was the army of the Byzantine state in its classical form (i.e. after the permanent loss of its Near Eastern and North African territories to the Arab conquests after 641 AD). This army was largely composed of semi-professional troops (soldier-farmers) based on the themata military provinces, supplemented by a small core of professional regiments known as the tagmata. Ibn al-Fakih estimated the strength of the themata forces in the East c. 902 at 85,000 and Kodama c. 930 at 70,000. This structure pertained when the empire was on the defensive, in the 10th century the empire was increasingly involved in territorial expansion, and the themata troops became progressively more irrelevant, being gradually replaced by 'provincial tagmata' units and an increased use of mercenaries.
Komnenian Byzantine army (1081–1204)
The Komnenian Byzantine army was named after the Komnenos dynasty, which ruled from 1081–1185. This was an army built virtually from scratch after the permanent loss of half of Byzantium's traditional main recruiting ground of Anatolia to the Turks following the Battle of Manzikert in 1071, and the destruction of the last regiments of the old army in the wars against the Normans in the early 1080s. It survived until the fall of Constantinople to the Western crusaders in 1204. This army had a large number of mercenary regiments composed of troops of foreign origin such as the Varangian Guard, and the pronoia system was introduced.
Palaiologan Byzantine army (1261–1453)
The Palaiologan Byzantine army was named after the Palaiologos dynasty (1261–1453), which ruled Byzantium from the recovery of Constantinople from the Crusaders until its fall to the Turks in 1453. Initially, it continued some practices inherited from the Komnenian era and retained a strong native element until the late 13th century. During the last century of its existence, however, the empire was little more than a city-state that hired foreign mercenary bands for its defence. Thus the Byzantine army finally lost any meaningful connection with the standing imperial Roman army.
This article contains the summaries of the detailed linked articles on the historical phases above, Readers seeking discussion of the Roman army by theme, rather than by chronological phase, should consult the following articles:
Strategy and tactics
Equipment & other
Some of the Roman army's many tactics are still used in modern-day armies today.