The Roman army (Latin: exercitus Romanus) was the armed forces deployed by the Romans throughout the duration of Ancient Rome, from the Roman Kingdom (753 BC–509 BC) to the Roman Republic (509 BC–27 BC) and the Roman Empire (27 BC–476 AD), and its medieval continuation, the Eastern Roman Empire. It is thus a term that may span approximately 2,205 years (753 BC–1453 AD), during which the Roman armed forces underwent numerous permutations in size, composition, organisation, equipment and tactics, while conserving a core of lasting traditions.
Early Roman army
Until c. 550 BC, there was no "national" Roman army, but a series of clan-based war-bands which only coalesced into a united force in periods of serious external threat. Around 550 BC, during the period conventionally known as the rule of king Servius Tullius, it appears that a universal levy of eligible adult male citizens was instituted. This development apparently coincided with the introduction of heavy armour for most of the infantry. Although originally low in numbers the Roman infantry was extremely tactical and developed some of the most influential battle strategies to date.
The early Roman army was based on a compulsory levy from adult male citizens which was held at the start of each campaigning season, in those years that war was declared. There were no standing or professional forces. During the Regal Era (to c. 500 BC), the standard levy was probably of 9,000 men, consisting of 6,000 heavily armed infantry (probably Greek-style hoplites), plus 2,400 light-armed infantry (rorarii, later called velites) and 600 light cavalry (equites celeres). When the kings were replaced by two annually elected praetores in c. 500 BC, the standard levy remained of the same size, but was now divided equally between the praetors, each commanding one legion of 4,500 men.
It is likely that the hoplite element was deployed in a Greek-style phalanx formation in large set-piece battles. However, these were relatively rare, with most fighting consisting of small-scale border-raids and skirmishing.