The earliest Roman army existed under the kings until around 550 B.C., when the king Servius Tullius instituted the first Roman military reforms. This army was quite similar to the armies of the early feudal era; a series of clan-based war-bands, usually comprised of loyalty-bound patrons serving under their client, who supplied them with the necessary tools for war, would come to the aid of the King when called for and which only coalesced into a united force in periods of serious external threat. During this era, ‘warfare’ consisted of chronic small-scale raiding and pillaging against neighboring peoples, and set-piece battles between tribes and peoples were quite rare.
The adoption of Greek-style hoplite equipment in the 6th and 5th century B.C. changed the way the Roman army was equipped, organized, and how it fought. The introduction of metal body-armor and new military tactics such as the phalanx created a differentiation in the citizen-body between those wealthy enough to pay for such equipment (known as the classis, or “class”) and those who were not and continued to serve as unarmored light infantry (the infra classem, or “beneath the class”). The Roman King Servius Tullius introduced the so-called ‘Servian centuriate organization’, in which property-owning male citizens were divided into five classes for military service based on their wealth and then organized into centuries of 100 men as sub-units of the greater Roman legion. During this era, the Greek phalanx was the common military formation and form of battle. Roman soldiers would have looked very similar to the Greek hoplite armies of Alexander and Philip. Tactics were no different from those of the early Greeks and battles were joined on flat terrain; spearmen would deploy themselves in tightly packed rows to form a shield wall with their spears pointing forwards. They then charged the enemy supported by javelin throwers and slingers; the cavalry pursued the enemy, sometimes dismounting to support infantry in dire situations. The phalanx was a cumbersome military unit to maneuver and the Romans found themselves easily defeated by the more adaptable and maneuverable forces of the mountain tribes such as the Volsci or Samnites in rough terrain. During this period, the standard levy of a single legion numbered 9,000 men, with 6,000 hoplites; 2,400 light infantry; and 600 cavalry.
As time went on, military developments made the phalanx an outdated formation and way of battle. During the era of the ‘mid-Republic’, from around 300 B.C. until Gaius Marius’s reforms in 107 B.C., the Roman army now drew up in three lines (triplex acies) consisting of small units (maniples) of 120 men, arrayed in chessboard fashion instead of the single, large mass of the phalanx. This gave it a much greater tactical strength and flexibility . Instead of fighting with long spears, the Romans were equipped with javelins and the short sword, or gladius. During this period the Roman army was often accompanied by a non-citizen formation of roughly equal size to the legion, the ala, which recruited from Rome’s Italian allies. These were about 150 autonomous states which were bound by a treaty of perpetual military alliance with Rome, and their sole obligation was to supply to the Roman army, on demand, a number of fully equipped troops up to a specified maximum each year. The Republican army of this period, like its earlier forebear, did not maintain standing or professional military forces, but levied them, by compulsory conscription, as required for each campaigning season and disbanded thereafter. ‘Warfare’ was an annual, almost ritualistic occurrence, and during the campaigning season the Roman army almost always found itself on a mission. During this era the army was split into two legions of 4,500 men each.
It was during this era that the central tactic of the Roman army was a shock infantry-charge, designed to put the enemy to flight as quickly as possible. The heavy-armed legionaries would advance at a measured pace towards the enemy line. When the gap was only around 15m, each successive line of legionaries would fling their two javelins (pila), draw their swords and break into a run, yelling their war-cry and charging into the enemy line. Smashing the enemy in the face with their shield-bosses, legionaries would use their swords to stab the enemy in the groin, belly, or face, often inflicting fatal wounds. Against tribal and unarmored enemies, the initial impact alone frequently resulted in the collapse of the enemy line. Against advanced enemies such as the Greeks, the initial impact would at least disrupt the enemy line and, in the ensuing melee, the Romans would benefit from their improved weaponry.
From the end of the Second Punic War in 201 B.C. onwards, the Republic’s army fought almost exclusively outside Italy in wars of conquest as Rome built a Mediterranean empire. This required men to remain under arms abroad for much longer periods, which was unpopular with the farmer-conscripts which made up the bulk of the earlier Roman army and who were concerned with the neglect of their lands. Their political pressure resulted in the passage of a law that conscripts could not be required to serve for more than 6 years consecutively. To circumvent this, it is believed that the army in this period recruited ever higher numbers of volunteers for long-term service. The most suitable such recruits were from the ranks of the landless lowest social class (proletarii) because they had no farms to tend and would be most attracted by the prospect of substantial gain in the form of booty. But these poor, despite being the largest social class, were excluded from service in the legions because they did not meet the minimum property-threshold required.
The consulship of Gaius Marius in 107 B.C. was the most dramatic reform to the Roman army since the development of its military organization by Servius almost 4 centuries earlier. Marius transformed the Roman army from a regularly-levied conscription force of farmer-soldiers into a professional force drawing from the poorest classes, thereby enabling Rome to field larger armies and providing employment for jobless citizens of the city of Rome. The legions were divided into cohorts, which had previously been temporary administrative units or tactical task forces of several maniples. Now the cohorts were ten permanent units, composed of 6 centuries of 100 men each. Tactics were not very different from the past, but their effectiveness was largely improved because of the professional training of the soldiers. Marius also eliminated the notion of allied legions; after his reforms all Italian legions would be regarded as Roman legions, and full Roman citizenship was open to all the regions of Italy. At the same time, the three different types of heavy infantry were replaced by a single, standard type armed with two heavy javelins, the short sword, chain mail (lorica hamata), helmet, and rectangular shield (scutum).
Now that we understand the history of the development of the Roman Republican army, lets discuss the extent of the zombie outbreak. During these times, communication between cities and often between the rural settlements surrounding a city could take days. Though surrounding the city of Rome was a number of other major city-states, such as the major city of Veii only 20-some miles to the North, it would take some time for a rider on horseback to spread the news of an infection or for a boat to go up or down the Tiber and the coast of Italy to warn the provinces. I’m of the mind that, once a zombie outbreak begins to grow in size, the ability to contain it becomes exponentially more difficult. A zombie infection in Rome or in the surrounding countryside could potentially grow too large to contain before the news of it could reach the proper authorities due to the slow speed of communication. Of course, we also need to bear in mind that people during this time weren’t especially spread across the landscape, but were rather clustered in population centers such as a city or in small manoral settlements. Throughout Republican Rome, most of the countryside was dotted with manors containing hundreds of people living and working on them. Because of this, you’d have concentrated outbreaks in these small areas, but then the zombies would need to wander the countryside for quite some time before finding more people to eat/infect. Thus, maybe the exponential growth of the undead rapidly infecting more and more people wouldn’t happen during this era because of the layout of the population, unless it started in a city.
How would the armies of Rome be able to react to and combat a zombie apocalypse? How would they do against an invasion of Gallic/Germanic zombies or against an outbreak in the lands around Rome? Where the zombies come from is of importance. If the infection begins in an outlying region of Rome, then it would be much easier for the Roman army to defeat it quickly. Either relying upon the armies of allied cities or upon a quick deployment and march to the infected area, the Roman army could conceivably overwhelm a zombie infection in its early stage, especially if it had failed to reach a major population center. In the case of a horde of Germanic zombies coming down from Germania or Gaul, the situation would likely be much different. The distance this horde would need to travel means that it would likely be considerably larger, especially considering that it would need to go through the heavily populated lands of the Etruscans.
The pre-Servian army would definitely get overrun. As it rarely came together as a unified force and was often made up of separate war-bands fighting autonomously, this army would likely be quickly overrun and defeated. Considering the difficulties of command and coordination during this time, organizing an effective defense against the zombies would be enormously challenging.
With the introduction of hoplite technology and the development of the Servian organization, the Roman army would probably have been able to withstand an invasion somewhat better, but likely not for long. Fighting as a phalanx, it would need to funnel the zombies into a kill-zone and then hold them off at a distance with its spears. The difficulty of maneuvering this army, and the fact that the Roman army was often comprised of a single legion of 6,000 men for most of the mid-Republic means that an effective defense against thousands or hundreds of thousands of zombies was simply impossible. Also important was the fact that this army was made up of farmer-soldiers; they were often hostile to the idea of campaigning for prolonged periods of time, and would want to return home after the campaign season. Defending against the zombie hordes would need a prolonged campaign, perhaps of years, and the people populating the army at this time were simply not up for the task. This army would likely have failed defending against the zombie hordes, and historically it did fail against invading Gauls in 390 B.C., when they sacked the city of Rome. The post-Servian hoplite army would, like the pre-Servian war-bands, be ill suited.
The army of the mid-Republic was more maneuverable, tactically flexible, and larger than the army of the early Republic, but even then it too faced the same challenges as the early Roman army. Made up mostly of conscripted farmers, it would not be able to campaign for prolonged periods of time. The style of fighting it conducted would also likely be dangerous against zombies: using the short sword, the Romans would need to get within striking distance of a zombie, meaning that they were exposed to bites and infection. Roman armor of this time was heavier than before, but even this was no guarantee against the hordes.
The best chance the Roman army had against zombies is thus likely the post-Marian army, which was able to campaign for long periods of time and which was much more flexible tactically to allow for maneuvers and quick formation changes to counter the hordes. By this period of time, Rome was in full control of Italy, and would likely be able to deploy a legion from anywhere to counter an incoming zombie horde with ease. Still, the zombies would pose a serious threat to the army, and the fact that warfare during this time relied on hand-to-hand combat meant that the Roman soldier would be much more threatened by the zombies than a soldier armed with a gun. The fact that the Roman army was made up of set-piece armies and tight formations means that it would be trying to fight an uncoordinated zombie horde as a single force; it would be near impossible using these tactics and formations to sweep the countryside, rout out all zombies, and defeat any stragglers. In battles against the tribes and peoples of the Roman hills which found in an uncoordinated manner like how zombie hordes would attack the Roman army, the Romans found themselves taking serious losses and in a serious disadvantage.
Most likely, Rome would be overrun.