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."[1] 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.

Phase I
The army was derived from obligatory annual military service levied on the citizenry, as part of their duty to the state. During this period, the Roman army would wage seasonal campaigns against largely local adversaries.
Phase II
As the extent of the territories falling under Roman control expanded and the size of the forces increased, the soldiery gradually became salaried professionals. As a consequence, military service at the lower (non-salaried) levels became progressively longer-term. Roman military units of the period were largely homogeneous and highly regulated. The army consisted of units of citizen infantry known as legions (Latin: legiones) as well as non-legionary allied troops known as auxilia. The latter were most commonly called upon to provide light infantry, logistical, or cavalry support.
Phase III
At the height of the Roman Empire's power, forces were tasked with manning and securing the borders of the vast provinces which had been brought under Roman control. Serious strategic threats were less common in this period and emphasis was placed on preserving gained territory. The army underwent changes in response to these new needs and became more dependent on fixed garrisons than on march-camps and continuous field operations.
Phase IV
As Rome began to struggle to keep control over its sprawling territories, military service continued to be salaried and professional for Rome's regular troops. However, the trend of employing allied or mercenary elements was expanded to such an extent that these troops came to represent a substantial proportion of the armed forces. At the same time, the uniformity of structure found in Rome's earlier military disappeared. Soldiery of the era ranged from lightly armed mounted archers to heavy infantry, in regiments of varying size and quality. This was accompanied by a trend in the late empire of an increasing predominance of cavalry rather than infantry troops, as well as a requirement for more mobile operations. In this period there was more focus (on all frontiers but the east) on smaller units of independently-operating troops, engaging less in set-piece battles and more in low-intensity, guerilla actions.