Coin showing (obverse) head of the late Roman emperor Julian
(ruled 361–363 AD) wearingdiadem
and (reverse) soldier bearing standard holding kneeling barbarian captive by the hair and legend VIRTUS EXERCITUS ROMANORUM ("Valour of the Roman army"). Gold solidus
The Roman army (Latin: exercitus Romanorum; Ancient Greek: στρατός/φοσσᾶτον Ῥωμαίων) is the terrestrial armed forces deployed by the Roman Kingdom (to c. 500 BC), theRoman Republic (500–31 BC), the Roman Empire (31 BC – 395/476 AD) and its successor, the East Roman or Byzantine Empire (395–1453). It is thus a term that spans approximately 2,000 years, during which the Roman armed forces underwent numerous permutations in composition, organization, equipment and tactics, while conserving a core of lasting traditions.
The development of the Roman army was divided into eight phases.
Early Roman army (to c. 300 BC)
The Early Roman army of the Roman kingdom and of the early republic (to c. 300 BC). During this period, when warfare chiefly consisted of small-scale plundering-raids, it has been suggested that the Roman army followed Etruscan or Greek models of organization and equipment. The early Roman army was based on an annual levy. According to Michael Ivanovich Rostovtzeff, “the army consisted of the entire population”. The infantry ranks were filled with the lower classes while the cavalry, equites or celeres, were left to the patricians. This was done so because the wealthier could afford horses. Moreover, the commanding authority during the regal period was the high king. Until the establishment of the Republic, as well as the consuls, the king assumed the role of “commander-in-chief”.  However, in 508 B.C. 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 latter being a reoccurring theme in many elements of the Roman Army. As mentioned before, the bulk of the army was citizens. These citizens did not get to choose which legion they wanted to be in. Any man “from ages 16-46 were selected by ballot” and assigned to a legion.  Up until the Roman military disaster of 390 B.C at the Battle of Allia, Rome’s army was organized in similarity 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. This determined which position he would be situated in the battle by the purchasing power of his military equipment. 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 (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) of the mid-Republican period (c. 300–88 BC).
During this period, the Romans, while maintaining the levy system, adopted the Samnite manipular organization 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, 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 complemented by a large number of volunteers willing to serve for much longer than the legal 6-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. During this period 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, Numidianlight 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 source 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 )
The Imperial Roman army (30 BC – 284 AD), when 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). The legions, consisting almost entirely of heavy infantry, numbered 25 of about 5,000 men each (total 125,000) under Augustus, increasing to a peak of 33 of about 5,500 (c. 180,000 men) by 200 AD under Septimius Severus. Legions continued to recruit Roman citizensmainly the inhabitants of Italy and Roman colonies until 212 AD. Regular annual conscription of citizens was abandoned and only decreed in emergencies (e.g. during the Illyrian revolt 6–9 AD). Legions were now flanked by theauxilia, 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 roughlycohort size, that is, about 500 men (125,000 men, or 50% of total army effectives). The number of regiments increased to about 400 under Severus, 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.
Late Roman army (284–476 AD) continuing as East Roman army (476–641 AD)
The Late Roman army (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 cohort or even smaller sizes. 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.