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The Imperial Roman Army is a favorite topic, and its broader organization has been well-treated elsewhere. However, I haven't been able to find a really thorough treatment of its mainstay - the legion - so I thought I'd provide one. Talking about the "Roman legion" is difficult, as the unit changed a great deal throughout the history of the empire. However, most consider the 1st and 2nd CE to be the high point of the Army, so I'll aim for the Trajanic legion of the late 1st century.

The military of the Imperial period was fully professional, made up of soldiers who served for at least 16 years and often more. There were 30 consecutively-numbered legions in Trajan's time, each with its own unique character and set of symbols and accolades. A typical legion boasted a full-strength complement of 5,500 men, including legionaries, officers, and support staff. Thus, the legions as a whole constituted some 160,000 men, fully half of the Roman military. The rest was made up of the Praetorian guard, the imperial fleet, and the auxilia, or auxiliary forces.

The largest part of the legion in Trajan's day was its 5,120 legionaries, who were organized into discrete sub-units within it. The smallest unit was the contubernia, which consisted of 8 soldiers. The men of a contubernia shared a pair of rooms when the legion was housed in a permanent camp, and a single tent when on campaign. Each contubernia was allotted a pack mule, which carried the tent and other common equipment when the legion was on the move. The contuberniae were grouped into 80-man centuria, or centuries, which were the legion's smallest fighting units. These centuries were paired in camp and on the march into units called maniples; however, the maniple was primarily a throwback to an earlier organizational system and was not particularly significant tactically. Instead, the centuries were grouped into ten cohorts. Cohorts II-X consisted of 480 men in 6 centuries, while cohort I consisted of 800 men grouped in 5 double-sized centuries.

Each century was commanded by a centurion, who was assisted by an optio, or junior officer, who would take control of the century if the centurion was incapacitated or killed in battle. The legion as a whole was commanded by the legatus legionis; this was a post given to members of the senatorial class as part of their political careers, so the quality of the legati varied widely. Beneath the legatus were six military tribunes, who were responsible for judicial and administrative duties. These officials were assisted and advised by the praefectus castrorum, the camp prefect, who ranked just below the senior tribune, and who would assume overall command of the legion if the senior tribune and legatus were absent. This prefect was normally a life-long soldier who had worked his way up through the centurionate and commanded the first cohort.

Each century had a tesserarius, who was in charge of guard and picket duties, a cornicen or horn-blower, and a signifer, who was responsible for the century's battle standard and kept track of pay and expenses. In addition, the legion as a whole had an aquilifer, who carried the legion's aquila (eagle) and imago (likeness of the emperor).

Auxilia units were deployed with the legion to support the legionary core by providing light skirmishers and cavalry. There were three auxilia unit types, and each came in two sizes: quingenaria and milliaria. These types were, in order of importance and prestige, the ala, cohors peditata, and cohors equitata. Alae were combat cavalry, divided into turmae, or squadrons. A squadron consisted of 30 cavalrymen, a commanding decurion, and an assistant officer called a duplicarius. An ala quingenaria consisted of 16 turmae, while an ala milliaria consisted of 24 turmae. A cohors peditata was an infantry unit made up of standard 80-man centuries; the quingenaria type was based on the standard legionary cohort, while the milliaria was based on the cohort I type. This type of unit was commanded by a praefectus cohortis and had the same officers and specialists as a legionary cohort. Finally, the cohors equitata was a mixed unit - a cohors equitata quingenaria had 4 turmae and 6 centuries, while a cohors equitata milliaria had 8 turmae and 10 centuries.

The legion's soldiers were extremely well protected. Each soldier wore a type of cuirass called the lorica segmentata, and a helmet called a galea which by Trajan's time had evolved to include neck and cheek guards. Leg armor was sacrificed for the sake of mobility, but the legionaries could easily protect their legs with the shield they carried, the scutum. This was a large, rectangular shield made of layered plywood and faced with leather; it had a metal boss in the center which protected the grip, as well as metal-rimmed edges and gilded or silvered decorations around the boss, which represented the thunderbolts of Jupiter.

The soldiers were also well armed. Each legionary carried a short stabbing sword called a gladius, supplemented by a pugio (dagger), and two pila (javelins.) A legionary could throw a pilum more than 60', and the weapon was designed to bend or break on impact, making it impossible for an enemy to turn it around and use it against the legion.

A legion deployed for battle in two lines - the first, from right to left, consisted of cohorts I-V, while the second, also right to left, was made up of cohorts VI-X. Each cohort was arranged with its three maniples side-by-side; the centuries within the maniples were deployed one behind the other, so the cohort arrayed for battle was three centuries wide and two deep. The first cohort would deploy in a square formation two centuries on a side, with the extra either backing up the other four or held in reserve. The best soldiers were usually assigned to cohorts I, III, V, VI, VIII, and X, meaning the center and ends of each line were the strongest. The cohorts were drawn up with a cohort's width between them, and the two lines were staggered so the cohorts of the second line overlapped the gaps in the first. Finally, any auxilia or numeri - specialists or irregulars - attached to the legion were deployed as skirmishers. Once the battle began in earnest, these skirmishers retreated through the gaps in the lines, which were then closed by the expedient of moving the rear century of each maniple up onto the battle line.

Roman doctrine included many different strategies for a legion engaged with the enemy, which took into account terrain, relative strengths of the two forces, and the strengths within the Roman line. The strategies were all aimed at turning the enemy's line, thus forcing them to fight the legionaries on more than one front, and more than likely breaking their formation to pieces. This well-developed doctrine, as well as the superior equipment and training of the legionaries, made the Imperial legion the mightiest fighting force of the ancient world.

I got most of the info for the essay from which this is condensed from Julian Bennett's Trajan, Gary Brueggeman's The Roman Army, Henry Parker's The Roman Legions, George Watson's The Roman Soldier, and Graham Webster's The Roman Imperial Army. All are highly recommended.
Throughout the history of the Roman Empire there have been so many reforms to the army that in order to understand where the Imperial Roman Legion came from, one must study entire history. The "classical" Roman Legionnaire image that we have today is the Imperial Roman Legionnaire, which stems from all the way back to the beginnings of Rome.

Rome was first settled by Greeks, known as the Etruscans. The Etruscans were Hellenistic Monarchs that stem back to before 600 B.C. Rome itself was founded around 600 B.C., and the Etruscans brought with them the classic Greek army. The army was rather chaotic, as they were entirely conscripts that were paid very little and respected even less. The army was made up of 12000 Hoplitai (Hoplites) and Phalanx, and was composed of:

  • 6400 Plebians (lower class)
  • 3200 Equites (middle class)
  • 1600 Patricians (upper class)
  • 160 Engineers (blacksmiths, medics etc.)
  • 160 Trumpeters

By the 4th Century B.C., Rome had broken away from the Etruscans and established themselves as an independent nation. After suffering defeat at the hands of the Gauls, and bearing the humiliation of having Rome sacked, the Romans were more than eager to introduce reforms into the army. The first step was to abandon the aging Hoplitai and Phalanx, and instead produce a more modern, and thoroughly Roman unit - the Legionnaire. The second step was to end the chaotic mass of conscripts, and instead produce an ordered mass of conscripts. The third and final step was to stop conscripting purely based on class, but instead based on age. The army was now divided into four ranks, and in each Legion there was:

  • 1200 Hastati (Young men)
  • 1200 Principes (Men in their prime)
  • 600 Triarii (Older men)
  • 1200 Velites (Youngest/poorest citizens)

The army was organised from front to rear as above, with the best equipped and most physically fit men taking the brunt of the attack, and the poorer equipped, older men as support. Each legion was also accompanied by 300 cavalry as support. Tactics were as follows: The Hastati, who wore a standard cuirass, carried a scutum (rectangle shield), a short sword and a javelin, would charge forward, skirmish with the enemy, then fall back to the Principes. They wore a heavy cuirass, carried a scutum, a spear and a short sword, but were better trained and had more experience than the Hastati. If the Hastati and the Principes broke, they would fall back to the Triarii, and finally the Velites. Although, as the Roman saying went, "It has come to the Triarii", meaning that the battle was virtually lost by this stage.

As the Roman Republic started to bloom, the came into contact with Hannibal and his Carthaginian Empire. Violent fighting erupted in Spain, and two key battles, Cannae and Trebia saw the decimation of Roman forces; although a key lesson was learnt. A young, ambitious Roman named Scipio was present at these battles, and saw the need for new tactics to be brought into the Roman army. While he did not change the way the army was formed, he brought in many of the Roman tactics and formations that were used throughout the Roman era and well into the medieval period. In a much smaller scope, it lead to the defeat of the Carthaginians, the sacking of Carthage and Scipio being entitled Scipio Africanus.

There was very little change until the 2nd Century B.C., the late Republic/early Principate. Numbers still hadn't changed, but formations and technology had. The Hastati still manned the front, but now wore bronze breastplate, or the wealthier ones wore chain shirts, and carried pilum and short javelins. The Principes mostly wore chain shirts, and carried a hasta (long spear), short sword and a similar rectangle shield as before. The Triarii and Velites generally still wore outdated armour, and carried a hasta and short sword. Previously, a legion had been divided into maniples, 120 men, but now it was divided into centuries, 60 men, with a centurion leading each one. Similarly, the cavalry were divided into turmae, consisting of 30 cavalry each, and were each lead by a decuriones. This made it easier to divide the legion and lead them into a new formation.

Until this point, the army had been entirely made from conscripts who were obliged to serve a six year service, but could remain afterwards if they so wished. They were taken from their lands without warning, expected to pay for their own equipment and training, and when they returned home, more often than not found their wife (if they had one) re-married and their land occupied by someone else. Marius Gracchus was the first to change this, first by introducing a census, second by introducing pay and third by allowing volunteers. The census was to be done once every three years, and made it so that army veterans could not be conscripted once again, and also raised the age limit to 16 (previously it had been 14). He also introduced pay, although it was a pittance, it helped the poorer conscripts to pay for their equipment; and upon finishing their term he made sure that they were given land to live upon. Finally, he allowed people to volunteer, but also lengthened the term conscripts served, enticing people to volunteer by making the volunteers' term shorter. Thus, the beginnings of a professional army had begun. Finally, Marius introduced "mules", where each soldier would carry on him a huge pack that contained food, water, a digging spade, a tent, and other necessites, so that base camp could be packed up and moved if need be.

The major reforms to the Roman army came under Julius Caesar and Augustus. Here we get the "classical" Roman Legionnaire. By this stage (60 B.C. - 14 A.D.) Legionnaires wore the "classical" lorica segmentata (splint mail) and lorica hamata (chain mail), wore the typical gallic helm and carried tower shields. Their weapons remained as pilum, short swords and small javelins. Major reforms had been carried out, and the army now consisted of a largely voluntary army. Pay was enough to purchase all needed equipment and then have enough to make a profit on (just), retired soldiers were always given land to return to, and a professional body was truly beginning to emerge. The army was completely reformed, numbers were changed and segments completely remodelled. Under Augustus, the Roman army stood at 28 legions strong, and was composed like this:

  • Contuberniun - 8 Men
  • Century - 10 Contuberniun (80 Men)
  • Maniple - 2 Centuries (160 Men)
  • Cohort - 6 Centuries (480 Men)
  • Legion - 10 Cohorts + 120 Horsemen (5240 Men*)

*1 Legion = 9 normal cohorts (9 x 480 Men) + 1 "First Cohort" of 5 centuries (but each century at the strength of a maniple, so 5 x 160 Men) + 120 Horsemen = 5240 Men

From this point on, few changes were made to the army, except updated technology, some changes in numbers and new formations. Still, this "classical" Legionnaire more or less persisted until the fall of Rome in A.D. 436, but was then carried on through the Byzantine Infantry, at least until they were consumed by Greek tradition. Thus, I shall now move on to famous Roman formations.

Key: L - Legionnaires, A - Archers, C - Cavalry, //-------L/A-------// = Century, ---C--- = Turmae
Note: There are more tactics than these, these are just the better known ones.

Three Line Defence

This is the most basic and ancient formation:

         //-------L-------//-------L-------//
         //-------L-------//-------L-------//
         //-------L-------//-------L-------//  
---C---  //-------L-------//-------L-------//  ---C---
---C---                                        ---C---
---C---  //-------A-------//-------A-------//  ---C---
---C---  //-------A-------//-------A-------//  ---C---
---C---                                        ---C---

Basically, the Legionnaires took the brunt of any attack, while the archers would shower the enemy on their approach. Cavalry would skirmish where possible.

Skirmish Formation

Like the three line defence, but staggered:

         //- - - - -L- - - - -//- - - - -L- - - - -//
         //- - - - -L- - - - -//- - - - -L- - - - -//
         //- - - - -L- - - - -//- - - - -L- - - - -// 
---C---  //- - - - -L- - - - -//- - - - -L- - - - -//  ---C---
---C---                                                ---C---
---C---  //- - - - -A- - - - -//- - - - -A- - - - -//  ---C---
---C---  //- - - - -A- - - - -//- - - - -A- - - - -//  ---C---
---C---                                                ---C---

Thus, the Legionnaires and archers are spread over a larger area. This gives them two advantages, firstly they are harder to hit with arrows, and secondly they can defend a larger area. The disadvantage, however, is that the line is easier to break through.

Wedge Formation

This is ancient tactic used to try and break enemy lines:

                    L   
                  L   L 
---C---         L       L         ---C---
---C---       L           L       ---C---
---C---     L               L     ---C--- 
---C---   L //------A------// L   ---C---
---C---     //------A------//     ---C---       
            //------A------//
            //------A------//

This is a purely offensive technique. The Legionnaires would form a wedge and charge forward, the archers running behind and shooting at the enemy, thought their effectiveness was dulled by the charge. The cavalry would run beside and skirmish where possible. The Legionnaires would run straight for the centre of an enemy defence, usually a three line defence, and with the tip would try to break their line and "wedge" into it. The Legionnaires would then push either direction, routing the unit, and the cavalry would pursue and harass the fleeing troops.

Turtle Shell Formation

Perhaps most famous, is the "Turtle Shell" formation, which looks like this:

LLLLLLLLLL
LLLLLLLLLL
LLLLLLLLLL
LLLLLLLLLL
LLLLLLLLLL
LLLLLLLLLL

The Legionnaires on the outside all crouch and face their shield outwards with their spears pointing outwards, whereas the Legionnaires in the centre crouch and face their shields upwards with their spears pointing up. The Legionnaires that were facing the direction they were going would then call out to move, and the body would move as one, all their shields interlocked, making them a veritable arrow fortress. Once they were close to a melee, the formation would break and they would fan out into the standard three line defence. All the units concerned here are Legionnaires, as archers had to remain behind, and cavalry, obviously, could not fit in the "shell".

Repel Cavalry Formation

As the name suggests, this was to repel cavalry:

//-------L-------//   //-------L-------//   //-------L-------//   //-------L-------//  
//-------L-------//   //-------L-------//   //-------L-------//   //-------L-------//

//-------A-------//-------A-------//             //-------A-------//-------A-------//

The Legionnaires would stand in two rows, the front row crouched down with their shield perpendicular to the ground and their spear pointing out between the interlocked shields, and the second row would stand with their shields at 45 degree angles to the first rows' shields, their spears held over the top of their shields. Archers would stand back and fire at the incoming cavalry, while the Legionnaires would take the charge with their spears, while being well protected by their shields. Cavalry skirmished if it was necessary, but generally were not included in this formation.

Sphere Formation

This formation was used to protect non-melee forces:

                L
             L AAA L
           L AAAAAAA L
         L  AAAAAAAAA  L
       L   AAAAAAAAAAA   L
         L  AAAAAAAAA  L
           L AAAAAAA L
             L AAA L
                L

Obviously, it would be more sphereish than this, but it is basically the same. The Leggionaires encircled the non-melee units, whether they be archers, dignitaries, civilians, or someone else that needs protection, and they held their shields close together with their spears pointing outwards. If there were archers in the centre, they would fire at any charging opponents.

Cannae Tactic

This is a formation/tactic that was used by the Carthaginians at Cannae. It slaughtered the Roman legions, and was adopted by Scipio Africanus:

//------L-------//------L-------//  ---C--- ---C--- ---C---  //------L-------//------L-------//
//------L-------//------L-------//  ---C--- ---C--- ---C---  //------L-------//------L-------//
//------L-------//------L-------//  ---C--- ---C--- ---C---  //------L-------//------L-------// 
                              //-------A-------//-------A-------//
                              //-------A-------//-------A-------//

This is how the formation started, and the enemy would charge at the cavalry in the centre. They would do this, as this is obviously the weakest point, for the cavalry are restricted, and cannot stand to take a charge, but, when the opponent was clost enough, the archers would split and the fasting moving cavalry would fall back:

//------L-------//------L-------//                       //------L-------//------L-------// 
//------L-------//------L-------//                       //------L-------//------L-------//
//------L-------//------L-------//                       //------L-------//------L-------//

           //-------A-------//    ---C--- ---C--- ---C---    //-------A-------//   
             //-------A-------//  ---C--- ---C--- ---C---  //-------A-------//
                                  ---C--- ---C--- ---C---

By this stage, the enemy would be too far through to stop the charge, and would continue through the gap, the archers shoot at them, the Legionnaires would crush them from both sides and the cavalry would pursue and harass the routing troops.


Sources:
Ancient Rome - Using Evidence by Pamela Bradley
http://webpages.charter.net/brueggeman/table-of-contents.html
http://members.tripod.com/~S_van_Dorst/legio.html
http://www.roman-empire.net/army/tactics.html

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