Structural history of the Roman military

The structural history of the Roman military concerns the major transformations in the organization and constitution of ancient Rome's armed forces, "the most effective and long-lived military institution known to history." From its origins around 800 BC to its final dissolution in AD 476 with the demise of the Western Roman Empire, Rome's military organization underwent substantial structural change. At the highest level of structure, the forces were split into the Roman Army and the Roman Navy, although these two branches were less distinct than in many modern national defense forces. Within the top levels of both army and navy, structural changes occurred as a result of both positive military reform and organic structural evolution. These changes can be divided into four distinct phases.

According to the historians Livy and Dionysius of Halicarnassus, writing at a far later date, the earliest Roman army existed in the 8th century BC. During this period Rome itself was probably little more than a fortified hilltop settlement and its army a relatively small force, whose activities were limited "mainly [to] raiding and cattle rustling with the occasional skirmish-like battle". Historian Theodor Mommsen referred to it as Rome's curiate army, named for its presumed subdivision along the boundaries of Rome's three founding tribes (Latin: curiae), the Ramnians, Tities and Luceres. This army's exact structure is not known, but it is probable that it loosely resembled a warrior band or group of bodyguards led by a chieftain or king. Mommsen believes that Roman military organization of this period was regimented by the "Laws of [the apocryphal] King [V]Italus" but these laws, though referred to by Aristotle, have been lost.

The army (Latin: legio) consisted, according to Livy, of exactly 3,000 infantry and 300 horsemen, one third from each of Rome's three founding tribes. Warriors served under six "leaders of division" (Latin: tribuni) who in turn served under a general, usually in the person of the reigning King. Mommsen uses philological arguments and references from Livy and others to suggest that the greater mass of foot-soldiers probably consisted of pilumni (javelin-throwers), with a smaller number possibly serving as arquites (archers). The cavalry was far smaller in number and probably consisted solely of the town's richest citizens. The army may also have contained the earliest form of chariots, hinted at by references to the flexuntes ("the wheelers").

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