Some time during the early Roman Republic the Roman army was split into two administrative units. Following the overthrow of the democracy, executive authority rested in the annual election of consul positions. There were five consuls, and each commanded one state. This could lead to competitiveness, and in the mid-Republican years when small armies began to be raised, such as during the early years of the First Punic War, command often went to one consul on alternate days for the entire army.

Every soldier bought and maintained their own equipment and there was no uniform dresscode. The younger, least experienced formed the velites, armed with a bundel of light javelins, a dagger (pugio) and a large round shield. Their special feature was wearing wolfskins. The next step in age and battle experience were called hastati, who wore some protective equipment and were armed with a sword (changed from the xiphos and kopis to the gladius), a javelin with throwing straps, a heavy javelin (pilum) and a large oval shield called a scutum.

The more experienced men of a legion were called Principes, they were similary armed but were more heavily armored than Hastati. The backbone of the Republican Army were called the Triarii, armed with long spears (hasta) and swords. They were the wealthy in Roman society and bought their own equipment. The Roman cavalry (equites) was in between light and heavy cavalry and relied less on ranged combat with javelins than close combat in the Greek fashion. Native Roman resources of cavalry were limited, so this branch consisted mostly of alae. Allied Italian states (alae) would send troops to fight alongside the legions in their own formations, but were escorted and commanded by Romans.

The first Roman wars were wars of expansion and defence, aimed at protecting Rome itself from neighbouring cities and nations by defeating them in battle. This sort of warfare characterized the early Republican Period when Rome was focused on consolidating its position in Italy, and eventually conquering the peninsula. Rome first began to make war outside the Italian peninsula in the Punic wars against Carthage. These wars, starting in 264 BC saw Rome become a Mediterranean power, with territory in Sicily, North Africa, Spain, and, after the Macedonian wars, Greece. One important point that must be understood is that the Rome did not conquer most nations outright, at least at first, but instead forced them into a submissive position as allies and client states. These allies supplied men, money, and supplies to Rome against other opponents.

Both in prestige and capability, the Roman navy was the smaller arm of the Roman military. Before the First Punic War in 264 BC there was no Roman navy to speak of as all previous Roman war had been fought in Italy. But the war in Sicily against Carthage, a great naval power, forced Rome to quickly build a fleet and train sailors. The first few naval battles of the First Punic War were disasters for Rome, and it was not until the invention of the Corvus, a grappling engine which made it easier for Romans to board the Carthaginian vessels, that Rome was able to win the war. This meant that Rome could use her superior army in naval combat, and was a significant shift away from the tactics of all other navies at the time.

The rural economy was permanently damaged in the decades following the war against Hannibal and the simultaneous Gallic uprising. Military manpower began to dwindle with the number of landholders and the equipment of the ones available decreased. The numbers actually required to defend Rome's growing territory increased. Suddenly the Cimbri and Teutoni appeared and annihilated several Roman armies. In this situation the consul Gaius Marius put forward the so-called Marian reforms. He lowered the taxlevel (census) for recruitment into the army and supplied the soldiers with equipment at public expenses. Compareable acts had only been done during the Second Punic War after the defeat at Cannae. It faced opposition in the senate because it abandoned permanently the mos maorum (established order of things) and changed military and social structure. On the other hand this led to the professional army defeating the threatening Cimbri and Teutoni.

The regular army of Romans enabled a constant warfare, while the Italian allies (Socii) were still bound to provide alae, half of the infantry and most of the cavalry. This supported the strong interest for equal rights (Roman citizenship) and after the assassination of Marcus Livius Drusus the Social War broke out. The professional Roman troops defeated the Italian militias in decisive engagements, but the Socii achieved their objectives with the Lex Julia and Lex Plautia Papiria. Although on the long run this military successful army could be viewed as a negative innovation for the republic. It was no longer bound to the senate and the people of Rome (Senatus Populusque Romanus), but profited directly from the achievements of their commanders.

In the enduring conflict between populares and optimates, with casualties on both sides, Lucius Cornelius Sulla was the first to use his military power in a coup to become lifetime dictator of Rome. Captured followers of the aged but influential Gaius Marius were killed.

A typical legion of Romans was usually accompanied by auxiliary troops. Before the Social War these auxiliary troops, called alae, were distributed by the Roman Socii and fought with the same equipment and formations as the legions. After the reforms auxiliaries were made up of foreigners from the provinces outside of Italy and could be as many as equal in numbers. They were granted Roman citizenship after completing their twenty-five years of service. This system of foreign auxiliaries allowed the post-Marian army to strengthen traditional weak points of the Roman system with foreign specialists. These were particularly skirmishers, missile troops and cavalry. Domestic cavalry was missing, as the wealthy class of equites took less and less interest in military affairs.

In order to police the growing amount of territory outside of Italy, Rome had organized it into provinces, starting with Sicily after ←the First Punic War. They were commanded by a military governor called a proconsul or propraetor. The proconsul had absolute authority in the province he governed, and even had the power to raise legions. Although this could be officially a requirement for policing their territory, ambitious proconsuls would easily abuse their power against the republic itself, like Gaius Julius Caesar.

It was not until the late Republic that the expansion of the Republic started meaning actual annexation of large amounts of territory, however in this period, civil war became an increasingly common feature. In the last century before the common era at least twelve civil wars and rebellions occurred. These were generally started by one charismatic general who refused to surrender power to the Roman Senate, which appointed generals, and so had to be opposed by an army loyal to the Senate. This pattern did not break until Octavian (later Caesar Augustus) ended it by becoming a successful challenger to the Senate's authority, and was made princeps (emperor). In the Principate claim to power via armed forces and public opinion created revolution and mutiny of the armed forces as methods of deposition for a new claimant. "We are accustomed to regard revolution as something essentially unconstitutional, an appeal from law to force; but under the Imperial system it was not unconstitutional; the government was, as has been said, an autocracy tempered by the legal right of revolution."

Rome was able to use her superior army in preference to her navy in most of the wars she fought afterwards. Rome's last major naval battle was fought between the Romans Octavian and Marcus Vipsanius Agrippa against Marc Antony at Actium.

The Roman navy was comprised completely of volunteers. All empire, Roman control over the Mediterranean coast meant that there were no non-Roman navies to fight and the marines and sailors of the fleet spent their time policing the mediterranean against the threat of pirates and patrolled the various major rivers in the empire.

Between the reigns of Augustus and Trajan the army became a trained professional one. Its core legionaries were Roman citizens who served for a minimum of twenty-five years. Augustus in his reign tried to eliminate the loyalty of the legions to the generals who commanded them. Since then every soldier had to take an oath of direct allegiance to the emperor. While the legions remained relatively loyal during Augustus' reign, under others they often took the power into their hands. The continuous increase of accorded Roman citizenship substantially changed the structure of Roman society, visible in the increasing number of emperors and rulers from the provinces. The interest in military service continued to decline in Italy and established a continuous trend for loss of the military's national identity. The majority of legionaries was born in the provinces and with time going by even commanding positions, originally reserved for native Italians, were assigned to them. Loyalty of the legions to the emperors degraded with time due to corruption and mistreatment. Much like during the late Republican era generals were wrestling for control of power with the strength of legions under their command.

At the beginning of the Imperial period the number of legions was 60, which Augustus more than halved to 28, numbering approximately 160,000 men. As more territory was conquered throughout the Imperial period, this fluctuated into the mid-thirties. At the same time, at the beginning of the Imperial period the foreign auxiliaries made up a rather small portion of the military, but their numbers continued to rise, so that by the end of the period of the Five Good Emperors they probably equalled the legionaries in number, giving a combined total of between 300,000 and 400,000 men in the Army service.

Under Augustus and Trajan, the army had become a highly efficient and thoroughly professional body, brilliantly led and staffed. To Augustus fell the difficult task of retaining much that Caesar had created, but on a permanent peace-time footing. He did so by creating a standing army, made up of 28 legions, each one consisting of roughly 5300 men. Additional to these forces, there was a similar number of auxiliary troops. Augustus also reformed the length of time a soldier served, increasing it from six to twenty-five years. The standard of a legion, the so-called aquila (eagle), was the very symbol of the unit's honour. The aquilifer was the man who carried the standard; he was almost as high in rank as a centurion. It was this elevated and honourable position that also made him the soldiers' treasurer in charge of the pay chest. A legion on the march relied completely on its own resources for weeks. In addition to his weapons and armour, each man carried a marching pack that included a cooking pot, some rations, clothes and any personal possessions. Furthermore, to make camp each night every man carried tools for digging as well as two stakes for a palisade. Weighed down by such burdens it is little wonder that the Roman soldiers were nicknamed 'Marius' Mules'.

The Imperial centralization of power in Rome was both help and hindrance to the expansion of the Empire. Under emperors secure from interior enemies, such as Augustus and Trajan, the military achieved great territorial gains. Rulers considered weaker, such as Nero and Domitian, were commonly followed by usurpators, often from the legions. Therefore all successful emperors accomplished loyalty of the imperial legions and the praetorian guards. The less secure an emperor was, the more were they required to stay in Rome for maintaining their direct command in state affairs. Direct actions along the borders were then carried out by generals whose successful campaigns were likely to increase popularity among the troops. Such posed a possible threat to the established emperor. This meant that expansion came often in leaps and bounds rather than a slow march. Many of the conquered territories during imperial reign were former client states. Rome started to consider their governance as degraded regimes. To prevent instability spreading to the empire armed interventions were required, often leading to outright annexation.

Lacking support for the emperor among his military meant that commanders could establish sole control of the legions they were responsible for. The Crisis of the Third Century developed with the murder of the emperor Alexander Severus in 235 AD. A large number of military usurpers felt enough public support, starting a series of civil wars similar to those at the end of the Republic. Beginning with the Crisis of the Third Century several military officers claimed themselves emperors and reigned over parts of the empire for months or days. The time was characterized by a Roman army that was as likely to be attacking itself as an outside invader. Ironically, while it was these usurpations that led to the break up of the Empire during the crisis, it was the strength of several frontier generals that helped reunify the empire through force of arms.

The last major reform of the Imperial Army was started under the reign of Diocletian, ending the Crisis of the Third Century, and took root under Constantine I with widespread effect. During the instability which had marked most of the century, the army had fallen in number and lost much of its ability to police and defend the empire. Diocletian quickly recruited a large number of men, increasing the number of legionaries from between 150,000 and 200,000 to between 350,000 and 400,000, although at the expense of the quality. The new system changed from a more linear arrangement along the border to a defence staggered in depth. The military was divided into limitanei, troops guarding the border, and well-equipped comitatenses as mobile reaction forces. Another important change started under Diocletian was the division of military authority and governance of the provinces.

The limitanei or riparian were border units in the armies of the late Roman Empire. They maintained the task of guarding the borders, intercepting small raiding parties and holding off invading armies until the comitatenses arrived. Historically significant is their appearance as part of military reforms in the late third century A.D. to strengthen the defensive capabilities. The first written reference to "limitanei" was in 363 A.D.

Although these military reforms brought a more effective defensive army it had downsides. Limitanei had to live in poor camp conditions along the borders and occupy the watchtowers. The comitatenses in contrast lived very comfortable in the interior cities. Modern historians suppose this to have affected a decline of discipline and morale among the limitanei, effecting the late Roman army as a whole. Considering equipment and skill other historians point out that limitanei were often able to bolster up comitatenses units without significant loss in combat strength.

Comitatenses(sing. comitatensis)were more mobile line troops, a large part was cavalry; furthermore there were second line troops, named pseudocomitatenses, former limitanei attached to the comitatus; palatinae, elite ('guard') units typically assigned to Magistri militum; and the scholae of actual palace guards, notably under the magister officiorum, a major court official of the Late Empire.