How a Roman Army was Organized During the Time Between Marius’s Military Reforms (107 B.C.) and the Professionalization of Military by Augustus (1 B.C.)
The organization of the Roman army during the Late Republic was based on Marius’s military developments. There was the legion, if ideal and fully stocked, of 6,000 men or at least about 3,000 men. As proconsul, or governor, of Gaul and Illyricum, Julius Caesar started out with four legions and at most, later on, possessed 12 undersized legions; on average commanding 30,000 men while often facing armies of hundreds of thousands of soldiers.
Typically, a field army would be divided into three lines before battle. These three lines, or triple acies, could be arrayed either horizontally (as three rows, wide) or vertically (as three columns, long) depending on the topography of the landscape and orientation of the enemy force. Cohorts from the rear could rotate to refresh the front, peel off to defend a flank attack, chase down a fleeing contingency of enemy, or even pull back and build a wall or any other objective away from the main battle line.
In addition to infantry and some specialized ranged units like archers, a legion consisted of cavalry, 300-500 horsemen, placed on the flanks of infantry lines, and were called “wings.” Sometimes, all the cavalry units were grouped altogether under the command of a single officer. They would engage in skirmishes, taunt defensively situated enemies, chase down fleeing enemies, swing around to bolster certain overwhelmed points during battle, or charge to break up a strong huddle of opponents. Outside of pitched battles, they would be tasked with reconnaissance missions. Roman cavalry were more valued for their mobility than their actual usefulness in combat and were often comprised of auxiliary troops, or conscripted foreigners and their horses.
As far as the infantry of a legion went, let us imagine 6,000 Roman foot soldiers (though in reality 5,500 of them were combatants while the remainder were standard bearers or offered non-combat support). Within a legion, there were sub-organized 10 “cohorts.” A cohort consisted of 600 men, typically. These 600 men of a cohort were further organized into three “maniples” of 200 men each, or two centuries. Each maniple consisted of two “centuries” of 100 men led by a “centurion.” These centuries were flexible units that could be moved to turn, advance, retreat and break off from the line to move around as ordered. At each stage of these subdivisions, an officer of rank led and took orders. The officers were men of high birth, patricians connected to the Rome’s Senate, but centurions were promoted based on ability and tenure.
So, the century, being the last subdivision of the Roman legion, is where we can find ourselves most practically on the Roman battlefield. The soldiers in a century could either stand in line, huddled close together, or staggered into a “quincunx” formation, or five-point cross section, which if looked at from above would resemble the spacing of dots on the five-side of a dice. The quincunx created greater depth and width in the formation, in essence forming a meat grinder.
Now picture yourself, a shirtless barbarian wearing nothing but rough pants and leather shoes with a crude iron sword in hand, and if lucky or from a strong tribe, a wooden shield wrapped over with a layer of thick animal hide. You’ve been fortunate enough to survive the initial onslaught of Roman javelins, or pila, thrown at you, but your two mates beside you were not. Now, as a wall of red Roman shields charges at you, you run forward and meet them with a clash. Banging your shoulder, you bounce off a four-foot-tall shield and get the wind knocked out of you but you quickly regain your footing. Three men forming a triangle oppose you. Fifteen feet in front of you is one Roman soldier; and in front of him to his left and right – and even closer to you – are two more armored Romans with high quality shields and swords. The soldier to your left is likely not focused on you – his shield guards his left as he attacks to his right – so you swing at him, but his shield deflects your attack and your sword bounces off of it. The clanging metal reverberates and numbs your shaking hand as the bones in your hand hurt for a moment. But that pain subsides once you feel the cool steel of a short sword slide into your side just beneath your ribcage. You die by the hand of a Roman soldier.
What if you had gone for the soldier on your right? You are a strong individual. Maybe you lock swords with him. Spittle flies from your grimace as you maniacally scream to overpower him. You want to drop your shield and bear down on him with both hands, but don’t let your guard down. Look who’s standing five feet away. The roman soldier in that shifted second line. That quincunx formation forms a pocket of death for an enemy infantry to step into. He steps forward and swipes at your neck. What’s the opposite of a bird’s eye view? Oh, well. Your vision turns to black as the last rivulet of blood drips onto the grass from your severed head. Your friend follows in your footsteps and meets the same fate. You are buried in a pile of your fallen brethren.
That formation combined with the initial ranged attack; and the superiority of Roman armor and weapons; and the historic, innate Roman courage and propensity for war, especially as infantry; and of course the training and competence of Roman officers to readily adapt to situations in the topography or opponent, allowed Roman armies like that of Julius Caesar’s to successfully take on enemy armies much larger in size to victory.