Changes

see: pg. 3.</ref>  The Roman Empire in the Late Republic and Imperial phases began to also adopt a professional army composed of many units that were able to fight at any given time of year.<ref> For further details of how the late Republic and Imperial Roman armies functions as a professional fighting force, see: Southern, Pat. 2007. ''The Roman Army: A Social and Institutional History''. Oxford: Oxford Univ. Press.</ref> Initially, both ancient Greece and Rome armies consisted of soldiers that were conscripted for short periods, similar to ancient Mesopotamia; however, this was not sustainable for the Romans as they expanded their realm. The Roman army of the Imperial period heavily relied on volunteers and eventually created a much larger military bureaucracy and system where many legions or units simultaneously existed and composed of many nationalities. Many more units and specialized roles developed in the Roman army, where non-Romans found the army as one potential way to work up the Roman social ladder. For example, many emperors were of non-Roman origin and had advanced using the military.<ref>For information about some of the non-Roman emperors within the Roman Empire, see: Plutarch, Robin Seager, and Rex Warner. 2005. ''Fall of the Roman Republic''. Harmondsworth, Middlesex, England: Penguin Books.</ref> A key development in this period was basing, on a permanent basis, legions in distant provinces and creating an elaborate system of forts and infrastructure that facilitated the presence of the army for long periods in distant areas. The presence of foreigners within Roman armies may have mitigated the presence of the army in places, helping to make their presence more tolerable. <ref>Southern, Pat. 2007. The Roman Army: A Social and Institutional History. Oxford: Oxford Univ. Press.</ref>. Nevertheless, the key basic structure utilized by the Neo-Assyrians , which made soldiers full-time and developing a true officer core, was largely maintained and was essentially continued by the Romans.