Information about tactics can be derived from accounts of battles, but the very military manuals known to have existed and to have been used extensively by commanders, have not survived. Perhaps the greatest loss is the book of Sextus Julius Frontinus. But parts of his work were incorporated in the records of the historian Vegetius.
The importance of the choice of ground is pointed out.
There is an advantage of height over the enemy and if you are pitting infantry against cavalry, the rougher the ground the better. The sun should be behind you to dazzle the enemy. If there is strong wind it should blow away from you, giving advantage to your missiles and blinding the enemy with dust.
In the battle line, each man should have three feet of space, while the distance between the ranks is given as six feet.
Thus 10'000 men can be placed in a rectangle about 1'500 yards by twelve yards, and it was advised not to extend the line beyond that.
The normal arrangement was to place the infantry in the centre and the cavalry on the wings. The function of the latter was to prevent the centre from being outflanked and once the battle turned and the enemy started to retreat the cavalry moved forward and cut them down. - Horsemen were always a secondary force in ancient warfare, the main fighting being done by the infantry.
Vegetius also stresses the need for adequate reserves. These could prevent an enemy from trying to envelope one's own forces, or could fend off enemy cavalry attacking the rear of the infantry.
Alternatively, they could themselves move to the sides and perform an enveloping maneuver against an opponent.
Unfortunately, due to a revolution of military tactic and formation there is no way that modern warfare is similar to Roman warfare. There has been a switch from convetional warfare to guerilla warfare in the past century and the rules of engagement have completely changed.
Rome developed an outstanding army. In one battle under Julius Caesar, the Romans defeated a force of over 250,000 Gauls with only 50,000 Roman troops. Such victories were due to intense trainting. A new recruit would have to train twice a day and march eighteen miles in full armor, three times a month. Each person had to be able to march twenty miles in five hours. The Roman commanders kept the troops busy in practicing battle formations and techniques, building roads, and building forts.
The army was highly structured. When the army would move into enemy territory, there were tasks to perform. They started the day before dawn packing up their tents and equipment. Their weapons weighed on average thirty pounds (fourteen kilograms) in total. Each group of eight soldiers was responsible to carry their own equipment, including tent, cooking utensils, anddigging equipment that weighed a total of fourty pounds (eighteen kilograms). They then formed into columns to march to their next location. During the day, cavalry scouts were sent out to check for ambushes. Scouts were also sent ahead to look for a good campsite.
When the main troops arrived, they set a guard and the rest began digging a rampart. To develop a rampart, troops dug a ditch around the camp piled up the dirt into a small hill. They then placed sharpened stakes on the top of the hill forming a palisade of sorts around the camp. The camp was laid out in the exact same format as a fort, so all the men were able to make their way around without any confusion.
Views of the Gallic enemies of Rome have varied widely. Some older histories consider them to be backward savages, ruthlessly destroying the civilization and "grandeur that was Rome." Some modernist views see them in a proto-nationalist light, ancient freedom fighters resisting the iron boot of empire. Often their bravery is celebrated as worthy adversaries of Rome. See the Dying Gaul for an example. The Gallic opposition was also composed of a large number of different peoples and tribes, geographically ranging from the mountains of Switzerland, to the lowlands of France, to the forests of the Rhineland, and thus are not easy to categorize. The term "Gaul" has also been used interchangeably to describe Celtic peoples farther afield in Britain and Scotland, adding even more to the diversity of peoples lumped together under this name. From a military standpoint however, they seem to have shared certain general characteristics: tribal polities with a relatively small and lesser elaborated state structure, light weaponry, fairly unsophisticated tactics and organization, a high degree of mobility, and inability to sustain combat power in their field forces over a lengthy period. Roman sources reflect on the prejudices of their times, but nevertheless testify to the Gauls fierceness and bravery.
Though popular accounts celebrate the legions and an assortment of charismatic commanders quickly vanquishing massive hosts of "wild barbarians", Rome suffered a number of early defeats against such tribal armies. As early as the Republican period (circa 390-387 B.C.), they had sacked Rome under Brennus, and had won several other victories such as the Battle of Noreia and the Battle of Arausio. The foremost Gallic triumph in this early period was "The Day of Allia"- July 18- when Roman troops were routed and driven into the Allia River. Henceforth, July 18 was considered an unlucky date on the Roman Calendar.
Some writers suggest that as a result of such debacles, the expanding Roman power began to adjust to this vigorous, fast-moving new enemy. The Romans began to phase out the monolithic phalanx they formerly fought in, and adopted the more flexible manipular formation. The circular hoplite shield was also enlarged and eventually replaced with the rectangular scutum for better protection. The heavy phalanx spear was replaced by the pila, suitable for throwing. Only the veterans of the triarrii retained the long spear- vestige of the former phalanx. Such early reforms also aided the Romans in their conquest of the rest of Italy over such foes as the Samnites, Latins and Greeks. As time went on Roman arms saw increasing triumph over the Gallics, particularly in the campaigns of Caesar. In the early imperial period however, Germanic warbands inflicted one of Rome's greatest military defeats, (the Battle of the Teutoburg Forest) which saw the liquidation of three imperial legions, and was to spark a limit on Roman expansion in the West. And it was these Germanic tribes in part (most having some familiarity with Rome and its culture, and becoming more Romanized themselves) that were to eventually bring about the Roman military's final demise in the West. Ironically, in the final days, the bulk of the fighting was between forces composed mostly of barbarians on either side.
Whatever their particular culture, the Gallic and Germanic tribes generally proved themselves to be tough opponents, racking up several victories over their enemies. Some historians show that they sometimes used massed fighting in tightly packed phalanx-type formations with overlapping shields, and employed shield coverage during sieges. In open battle, they sometimes used a triangular "wedge" style formation in attack. Their greatest hope of success lay in 4 factors: (a) numerical superiority, (b) surprising the Romans (via an ambush for example) or in (c) advancing quickly to the fight, or (d) engaging the Romans over heavily covered or difficult terrain where units of the fighting horde could shelter within striking distance until the hour of decision, or if possible, withdraw and regroup between successive charges.
Most significant Gallic and Germanic victories show two or more of these characteristics. The Battle of the Teutoburg Forest contains all four: surprise, a treacherous defection by Arminius and his contingent, numerical superiority, quick charges to close rapidly, and favorable terrain and environmental conditions (thick forest and pounding rainstorms) that hindered Roman movement and gave the warriors enough cover to conceal their movements and mount successive attacks against the Roman line.
Weaknesses in organization and equipment. Against the fighting men from the legion however, the Gauls faced a daunting task. Individually, in single combat, the fierce Gallic warrior could probably more than hold his own against a Roman. In massed fighting however, the Gauls' rudimentary organization and tactics fared poorly against the well oiled machinery that was the Legion. The fierceness of the Gallic charges is often commented upon by some writers, and in certain circumstances they could overwhelm Roman lines. Nevertheless the in-depth Roman formation allowed adjustments to be made, and the continual application of forward pressure made long-term combat a hazardous proposition for the Gauls.
Flank attacks were always possible, but the legion was flexible enough to pivot to meet this, either through sub-unit maneuver or through deployment of lines farther back. The cavalry screen on the flanks also added another layer of security, as did nightly regrouping in fortified camps. The Gauls and Germans also fought with little or no armor and with weaker shields, putting them at a disadvantage against the legion. Other items of Roman equipment from studded sandals, to body armor, to metal helmets added to Roman advantages. Generally speaking, the Gauls and Germans needed to get into good initial position against the Romans and to overwhelm them in the early phases of the battle. An extended set-piece slogging match between the lightly armed tribesmen and the well organized heavy legionaries usually spelled doom for the tribal fighters. Caesar's slaughter of the Helvetti near the Saône River is just one example of tribal disadvantage against the well-organized Romans, as is the victory of Germanicus at the Weser River and Agricola against the Celtic tribesmen of Caledonia (Scotland) circa 84 A.D.
Weaknesses in logistics. Roman logistics also provided a trump card against Germanic foes as it had against so many previous foes. Tacitus in his Annals reports that the Roman commander Germanicus recognized that continued operations in Gaul would require long trains of men and material to come overland, where they would be subject to attack as they traversed the forests and swamps. He therefore opened sea and river routes, moving large quantities of supplies and reinforcements relatively close to the zone of battle, bypassing the dangerous land routes. In addition, the Roman fortified camps provided secure staging areas for offensive, defensive and logistical operations, once their troops were deployed. Assault roads and causeways were constructed on marshy ground to facilitate maneuver, sometimes under direct Gallic attack. These Roman techniques repeatedly defeated their Germanic adversaries. While Germanic leaders and fighters influenced by Roman methods sometimes adapted them, most tribes did not have the strong organization of the Romans. As German scholar Hans Delbruck notes in his "History of the Art of War":
The Gallics also demonstrated a high level of tactical prowess in some areas. Gallic chariot warfare for example, showed a high degree of integration and coordination with infantry, and Gallic horse and chariot assaults sometimes threatened Roman forces in the field with annihilation. At the Battle of Sentinum for example, c. 295 BC, the Roman and Campanian cavalry encountered Gallic war-chariots and were routed in confusion- driven back from the Roman infantry by the unexpected appearance of the fast-moving Gallic assault. The discipline of the Roman infantry restored the line however, and a counterattack eventually defeated the Gallic forces and their allies.
The accounts of Polybius leading up to the Battle of Telamon, c. 225 BC mention chariot warfare, but it was ultimately unsuccessful. The Gauls met comprehensive defeat by the Roman legions under Papus and Regulus. Chariot forces also attacked the legions as they were disembarking from ships during Caesar's invasion of Britain, but the Roman commander drove off the fast-moving assailants using covering fire (slings, arrows and engines of war) from his ships, and reinforcing his shore party of infantry to charge and drive off the attack. In the open field against Caesar, the Gallic/Celtics apparently deployed chariots with a driver and an infantry fighter armed with javelins. During the clash, the chariots would drop off their warriors to attack the enemy and retire a short distance away, massed in reserve. From this position they could retrieve the assault troops if the engagement was going badly, or apparently pick them up and deploy elsewhere. Caesar's troops were discomfited by one such attack, and he met it by withdrawing into his fortified redoubt. A later Gallic attack against the Roman camp was routed.
It should be noted also that superb as the Gallic fighters were, chariots were already declining as an effective weapon of war in the ancient world with the rise of mounted cavalry. At the battle of Mons Grapius in Caledonia (circa 84AD), Celtic chariots made an appearance. However they were no longer used in an offensive role but primarily for pre-battle show- riding back and forth and hurling insults. The main encounter was decided by infantry and mounted cavalry.
Superior Gallic mobility and numbers often troubled Roman arms, whether deployed in decades-long mobile or guerrilla warfare or in a decisive field engagement. The near defeat of Caesar in his Gallic campaign confirms this latter pattern, but also shows the strengths of Roman tactical organization and discipline. At the Battle of the Sabis river, (see more detailed article) contingents of the Nervii, Atrebates, Veromandui and Aduatuci tribes massed secretly in the surrounding forests as the main Roman force was busy making camp on the opposite side of the river. Some distance away behind them, slogged two slow moving legions with the baggage train. Engaged in foraging and camp construction the Roman forces were somewhat scattered. As camp building commenced, the barbarian forces launched a ferocious attack, streaming across the shallow water and quickly assaulting the distracted Romans. This incident is discussed in Caesar's Gallic War Commentaries.
So far the situation looked promising for the warrior host. The 4 conditions above were in their favor: (a) numerical superiority, (b) the element of surprise, (c) a quick advance/assault, and (d) favorable terrain that masked their movements until the last minute. Early progress was spectacular as the initial Roman dispositions were driven back. A rout looked possible. Caesar himself rallied sections of his endangered army, impressing resolve upon the troops. With their customary discipline and cohesion, the Romans then began to drive back the barbarian assault. A charge by the Nervi tribe through a gap between the legions however almost turned the tide again, as the onrushing warriors seized the Roman camp and tried to outflank the other army units engaged with the rest of the tribal host. The initial phase of the clash had passed however and a slogging match ensued. The arrival of the two rear legions that had been guarding the baggage reinforced the Roman lines. Led by the 10th Legion, a counterattack was mounted with these reinforcements that broke the back of the barbarian effort and sent the tribesmen reeling in retreat. It was a close run thing, illustrating both the fighting prowess of the tribal forces, and the steady, disciplined cohesion of the Romans. Ultimately, the latter was to prove decisive in Rome's long fought conquest of Gaul.
As noted above, the fierce charge of the Germanics and their individual prowess is frequently acknowledged by several ancient Roman writers. The Battle of Gergovia however demonstrates that the Gallic/Germanics were capable of a level of strategic insight and operation beyond merely mustering warriors for an open field clash. Under their war leader Vercingetorix, the Gallics pursued what some modern historians have termed a "persisting" or "logistics strategy" - a mobile approach relying not on direct open field clashes, but avoidance of major battle, "scorched earth" denial of resources, and the isolation and piecemeal destruction of Roman detachments and smaller unit groupings. When implemented consistently, this strategy saw some success against Roman operations. According to Caesar himself, during the siege of the town of Bourges, the lurking warbands of Germans were:
Caesar countered with a strategy of enticing the Germanic forces out into open battle, or of blockading them into submission.
At the town of Gergovia, resource denial was combined with concentration of superior force, and multiple threats from more than one direction. This caused the opposing Roman forces to divide, and ultimately fail. Gergovia was situated on the high ground of a tall hill, and Vertcingeorix carefully drew up the bulk of his force on the slope, positioning allied tribes in designated places. He drilled his men and skirmished daily with the Romans, who had overrun a hilltop position, and had created a small camp some distance from Caesar's larger main camp. A rallying of about 10,000 disenchanted Aeudan tribesmen (engineered by Vertcingeroick's agents) created a threat in Caesar's rear, including a threat to a supply convoy promised by the allied Aeudans, and he diverted four legions to meet this danger. This however gave Verctinorix's forces the chance to concentrate in superior strength against the smaller two-legion force left behind at Gergovia, and desperate fighting ensued. Caesar dealt with the rear threat, turned around and by ruthless forced marching once again consolidated his forces at town. A feint using bogus cavalry by the Romans drew off part of the Gallic assault, and the Romans advanced to capture three more enemy outposts on the slope, and proceeded towards the walls of the stronghold. The diverted Gallic forces returned however and in frantic fighting outside the town walls, the Romans lost 700 men, including 46 centurions.
Caesar commenced a retreat from the town with the victorious Gallic warriors in pursuit. The Roman commander however mobilized his 10th Legion as a blocking force to cover his withdrawal and after some fighting, the tribesmen themselves withdrew back to Gergovia, taking several captured legion standards. The vicious fighting around Gergovia was the first time Caesar had suffered a military reverse, demonstrating the Germanic martial valor noted by the ancient chroniclers. The hrd battle is referenced by the Roman historian Plutarch, who writes of the Averni people showing visitors a sword in one of their temples, a weapon that reputedly belonged to Caesar himself. According to Plutarch, the Roman general was shown the sword in the temple at Gergovia some years after the battle, but he refused to reclaim it, saying that it was consecrated, and to leave it where it was.
The Germanics were unable to sustain their strategy however, and Vertcingeroix was to become trapped in Alesia, facing not divided sections or detachments of the Roman Army but Caesar's full force of approximately 70,000 men (50,000 legionnaires plus numerous additional auxiliary cavalry and infantry). This massive concentration of Romans was able to besiege the fortress in detail and repulse Gallic relief forces, and it fell in little more than a month. Vertcingeroick's overall persisting logistics policy however, demonstrates a significant level of strategic thinking. As historian A. Goldsworthy (2006) notes: "His [Vercingetorix's] strategy was considerably more sophisticated than that employed by Caesar's earlier opponents.." At Alesia this mobile approach became overly static. The Gauls gave battle at a place where they were inadequately provisioned for an extended siege, and where Caesar could bring his entire field force to bear on a single point without them being dissipated, and where his lines of supply were not effectively interdicted. At Gergovia by contrast, Caesar's strength was divided by the appearance of another Germanic force in his rear (the Aeudans)- threatening his sources and lines of supply. Together with a strong defensive anvil, (the town) supported by an offensive hammer (the open field forces), and coupled with previous resource denial pressure over time, the Romans were forced to retreat, and the Germanics secured a victory. As one historian notes about the persisting strategy:
In their battles against a wide variety of opponents, Rome's ruthless persistence, greater resources and stronger organization wore down their opponents over time. In Spain, resources were thrown at the problem until it yielded over 150 years later- a slow, harsh grind of endless marching, constant sieges and fighting, broken treaties, burning villages and enslaved captives. As long as the Roman Senate and its successors were willing to replace and expend more men and material decade after decade, victory could be bought through a strategy of exhaustion.
The systematic wastage and destruction of enemy economic and human resources was called vastatio by the Romans. Crops and animals were destroyed or carried off, and local populaces were massacred or enslaved. Sometimes these tactics were also used to conduct punitive raids on barbarian tribes which had performed raids across the border. In the campaigns of Germanicus, Roman troops in the combat area carried out a "scorched earth" approach against their Germanic foes, devastating the land they depended on for supplies. "The country was wasted by fire and sword fifty miles round; nor sex nor age found mercy; places sacred and profane had the equal lot of destruction, all razed to the ground.." (Tacitus, Annals). The Roman "grind down" approach is also seen in the Bar Kokba Jewish revolt against the Romans. The Roman commander Severus, avoided meeting the hard-fighting Jewish rebels in the open field. Instead he relied on attacking their fortified strongpoints and devastating the zone of conflict in a methodical campaign. This "attritional" aspect of the Roman approach to combat contrasts with the notion of brilliant generalship or tactics sometimes seen in popular depictions of the Roman infantry.
Some historians note however that Rome often balanced brutal attrition with shrewd diplomacy, as demonstrated by Caesar's harsh treatment of Gallic tribes that opposed him, but his sometimes conciliatory handling of those that submitted. Rome also used a variety of incentives to encourage cooperation by the elites of conquered peoples, co-opting opposition and incorporating them into the structure of the empire. This carrot and stick approach forms an integral part of "the Roman way" of war.