Altar of Domitius Ahenobarbus, c. 122 BC; the altar shows two Roman infantrymen equipped with long scuta and a cavalryman with his horse. All are shown wearing chain mail armour.

Following the end of a term as praetor or consul, a Senator might be appointed by the Senate as a propraetor or proconsul (depending on the highest office held before) to govern a foreign province. More junior officers (down to but not including the level of centurion) were selected by their commanders from their own clientelae or those recommended by political allies among the Senatorial elite.[191]

Under Augustus, whose most important political priority was to place the military under a permanent and unitary command, the Emperor was the legal commander of each legion but exercised that command through a legatus (legate) he appointed from the Senatorial elite. In a province with a single legion, the legate commanded the legion (legatus legionis) and also served as provincial governor, while in a province with more than one legion, each legion was commanded by a legate and the legates were commanded by the provincial governor (also a legate but of higher rank).[194]

During the later stages of the Imperial period (beginning perhaps with Diocletian), the Augustan model was abandoned. Provincial governors were stripped of military authority, and command of the armies in a group of provinces was given to generals (duces) appointed by the Emperor. These were no longer members of the Roman elite but men who came up through the ranks and had seen much practical soldiering. With increasing frequency, these men attempted (sometimes successfully) to usurp the positions of the Emperors who had appointed them. Decreased resources, increasing political chaos and civil war eventually left the Western Empire vulnerable to attack and takeover by neighboring barbarian peoples.[195]

Less is known about the Roman navy than the Roman army. Prior to the middle of the 3rd century BC, officials known as duumviri navales commanded a fleet of twenty ships used mainly to control piracy. This fleet was given up in 278 AD and replaced by allied forces. The First Punic War required that Rome build large fleets, and it did so largely with the assistance of and financing from allies. This reliance on allies continued to the end of the Roman Republic. The quinquereme was the main warship on both sides of the Punic Wars and remained the mainstay of Roman naval forces until replaced by the time of Caesar Augustus by lighter and more maneuverable vessels.[196]

As compared with a trireme, the quinquereme permitted the use of a mix of experienced and inexperienced crewmen (an advantage for a primarily land-based power), and its lesser maneuverability permitted the Romans to adopt and perfect boarding tactics using a troop of about 40 marines in lieu of the ram. Ships were commanded by a navarch, a rank equal to a centurion, who was usually not a citizen. Potter suggests that because the fleet was dominated by non-Romans, the navy was considered non-Roman and allowed to atrophy in times of peace.[196]

Information suggests that by the time of the late Empire (350 AD), the Roman navy comprised several fleets including warships and

By the 3rd century BC, the Romans abandoned the hoplite formation in favor of a more flexible system in which smaller groups of 120 (or sometimes 60) men called maniples could maneuver more independently on the battlefield. Thirty maniples arranged in three lines with supporting troops constituted a legion, totalling between 4,000 and 5,000 men.[175][176]

The early Republican legion consisted of five sections, each of which was equipped differently and had different places in formation: the three lines of manipular heavy infantry (hastati, principes and triarii), a force of light infantry (velites), and the cavalry (equites). With the new organization came a new orientation toward the offensive and a much more aggressive posture toward adjoining city-states.[175][176]