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Evolution of the Roman Army
Steve Cabral

April 2014

The Roman Army holds the distinction of being the longest continually serving force known in military history. It was designed to defend the city state in her early days and armed and equipped in the Greek phalanx style common in the Mediterranean world. The early army is known as the Servian Army and was a citizen force of all males 17-60 with an age 46 cut-off for non-garrison service. Wealth determined place in the ranks with the richest going to the cavalry (equites). Eventually the Equites became the Equestrian Order between the Plebian (lower) Order and Patrician (Noble) Order.

The bulk of citizens became heavy infantry armed with the pike (sarissa) and wearing square bronze chest and back plates, and the poorest became the unarmored skirmishers. The heavy infantry was organized into three ranks from front to rear: Hastati (Youngest and greenest), Principes (veterans) and Triarii (oldest veterans and semi-elite). The Rorarii appear to have been a reserve held with the Triaii, possibly a medium infantry of older ex-velites. Accensi (Latin: singular) appear to be a rear rank reserve of servants from the fifth rank of citizens who fought if things were desperate. Ahead of the army were Velites, poor citizens who skirmished with javelins against an advancing force, eventually fading through the heavy infantry ranks as the main battle was joined. This army served under the Roman kings from the 6th to the 4th century BC. Its main opponents were Gauls, Etruscans, Greek colonists, Campanians, Bruttians and Samnites - all from north or south Italy.

The next incarnation of the army was the Camillan army alleged (but unlikely) to have been established by Fluvius Camillus. The legions (Latin: Legio) were reorganized into a more compact organization. The Hastati were given a semi-rectangular shield and re-armed with the Pila, the heavy long untempered pointed javelin thrown before contact. The Rorarii and Ascensii both disappeared from the army. They were still about against Pyrrhus and gone by the Second Punic War. The success of the Hastati caused the principes to be rearmed as the hastati (probably as the hastati aged they became principes with their pilum retained). The triarius maintained the long spear and wore chainmail; the velites were unchanged. Cavalry strength was set at 300 men. The short sword Gladius Hispanicus was adopted during the Second Punic War.

This army conquered the Mediterranean world from Spain to Syria. In this period a Roman Legion was paired with an allied legion, both armed the same. Hirelings from other city states/barbarian tribes could be engaged to fight in their native way under their own leadership; Gallic cavalry, Cretan archers and Balearic slingers were highly prized. After the wars came to an end the late 2nd century saw a number of disastrous encounters with Germans crossing into Northern Italy from the Alps. The Battle of Arausio 105BC saw 80,000 Romans killed in one day - a bigger defeat than Cannae. With this defeat Gaius Marius took charge of Rome and altered the army irrevocably.

Rome started as a city-state as in Classical Greece. With the expulsion of the Tarquin kings in 509BC a Republic was established. Surrounded by enemies, she slowly expanded her borders over the centuries until Rome suddenly found herself an empire ruled by an unwritten constutution that was not meant to cope with such a large land area. Slavery destroyed the independent free farmers who formed the backbone of Rome’s armies and turned them into urban poor who needed a welfare state to keep them from turning restive. Harsh measures against non-citizens caused discontent in provinces leading to the Social War in which Italian city-states rose against Rome demanding citizenship rather than independence, and also led to the Spartacus slave revolt. The age of the dictator was at hand and arose in the late 1st Century under Gaius Marius, a non-patrician brilliant general who became rich in Spain, Sulla his patrician subordinate, Sulla’s subordinate the non-patrician Pompey, and Marius’s patrician nephew Julius Caesar.

The Senate had to react after its army was destroyed at Arausio. Rome always found herself with limitless manpower, and Senators were shocked to find that it was gone. Marius assumed the consulship and promised the Senate he would have no problem assembling an army. He then recruited from the Censii (head count) - the poor of Rome. Idle young men flocked to Marius, forever giving him and his family (the Caesars) the poor's votes and loyalty. He reorganized the legions (Legio) into 10 cohorts - nine of 480 men, one of 600 men and a small 120-man cavalry contingent. Each cohort had six 80-man centuries (the missing 20 men are believed by some to be the old rorarii who served as servants and were promoted into the army as casulties caused openings). The 7th and 9th cohort were the most junior and the 1st and 6th the most veteran in quality. The troops were armored in mail as the triarius had been and all armed with the semi-rectangular shield, bronze helmet, Gladius Hispanicus and two pila.

At Vercellae, Marius, Sextus Julius Caesar and Sulla shattered the German army, killing 90,000 to 160,000 combatants and then hitting the German camp taking the woman and children as slaves. The new Marian army was a success, but Marius took all of the credit for victory and thus alienated his two subordinates, eventually starting a string of civil wars that (except for the Gallic Wars) saw the Marian legions fight other legions more often than against barbarians and foreigners. The Marian legion was reformed under Augustus Caesar after he became Emperor, making it a professional army with a vastly different Auxilla.

Under Augustus the 10 cohorts and legionary cavalry were unaltered (except for an improved bronze helmet) until they received their iron helmets, segmented plate armor and rectangular shields after 30 AD. Eastern armies of Rome kept chainmail armor. In many Legions one in four of the men carried slings and a wedge formation became more common. The Auxilla were formed into regular combat units. Infantry was organized as cohorts and equipped with a shield similar to that of the Camillian legions and with a javelin rather than the specialized pila. Full cohorts of archers (Sagitarius) were used to replace the light infantry Cretan archers. Their uniform varied depending on origin, with Easterners wearing more modest clothing. The Auxilla cavalry had a hexagonal shield, chainmail (some scale used especially after Nero) and javelin; a long sword called a Spatha was eventually adopted. Natives were occassionally hired on a short-term basis but not as heavily relied upon as in Marian and earlier armies.

This change caused an evolution in battle tactics, the Auxilla softened up the enemy force much as the Velites had once done. Their open order gave them more flexibilty than the Legio but less staying power. After intense pressure they would withdraw behind the heavies who would close in for a decision. Unlike the early Auxilla they were powerful enough to fight and win on their own as they did versus the Caledonians in Agricola’s British campaign. Auxillia were paid as regular troops, served 20 years, and received full citizenship on discharge. Both Marian and Imperial Legions had artillery batteries of light bolt throwers and heavier rock throwers. Staffing of batteries is unclear but probably came from 1st Cohort.

After 250AD the Imperial Legions deteriorated greatly in quality. Cavalry became the dominant arm, the infantry began shedding armor and barbarian hirelings became more important than ever. After Adrianople in 378AD cavalry would remain the arm of decision until Swiss pike blocks, English longbows and early firearms became dominant in the late middle ages and early modern period.

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