Roman Tactics

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.

It was recommended that if your cavalry was weak it was to be stiffened with lightly armed foot soldiers.

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 manoeuver against an opponent.

The position to be taken up by the commander was normally on the right wing.

The tortoise was a essentially defensive formation by which the legionaries would hold their shields overhead, except for the front rows, thereby creating a kind of shell-like armour shielding them against missiles from the front or above.

The wedge was commonly used by attacking legionaries, - legionaries formed up in a triangle, the front 'tip' being one man and pointing toward the enemy, - this enabled small groups to be thrust well into the enemy and, when these formations expanded, the enemy troops were pushed into restricted positions, making hand-to-hand fighting difficult. This is where the short legionary gladius was useful, held low and used as a thrusting weapon, while the longer Celtic and Germanic swords became impossible to wield.

The saw was opposite tactic to the wedge. This was a detached unit, immediately behind the font line, capable of fast sideways movement down the length of the line to block any holes which might appear to develop a thrust where there might be a sign of weakness. In the case of two Roman armies fighting each other in a civil war, one might say that the 'saw' inevitably was the response to a 'wedge' by the other side.

The skirmishing formation was a widely spaced line up of troops, as opposed to the tighter packed battle ranks so typical of legionary tactics. It allowed for greater mobility and would have found many uses in the tactical handbooks of Roman generals.

The order to repel cavalry brought about a the following formation. The first rank would form a firm wall with their shields, only their pila protruding, forming a vicious line of glistening spearheads ahead of the wall of shields. A horse, however well trained, could hardly be brought to break through such a barrier. The second rank of the infantry would then use its spears to drive off any attackers whose horses came to a halt. This formation would no doubt prove very effective, particularly against ill-disciplined enemy cavalry.

The orb is a defensive postition in the shape of a circle taken by a unit in desperate straits. It allows for a reasonably effective defence even if parts of an army have been divided in battle and would have required a very high level discipline by the individual soldiers.

Here are seven specific instructions by Vegetius regarding the layout before battle: 

All these tactics have the same purpose , that of breaking the enemy battle line. If a flank can be turned, the the strong centre has to fight on two fronts or is forced to fight in a restricted space. Once an advantage like this has been gained it is very difficult to correct the situation. Even in the highly trained Roman Army it would have been difficult to change tactics during the course of the battle and the only units which can be successfully deployed are those in the reserves or that part of the line not yet engaged. Thus the most important decision a general had to make concerned the disposition of the troops. If a weakness could be detected in the enemy line, it was exploited by using a stranger force to oppose it. Likewise, it was necessary to disguise one's battle line - even troops were disguised to delude the enemy. Often the very size of the army was skillfully hidden, troops packing tightly together to make it appear small, or spreading out to appear large. There were also many examples of surprise tactics made by detaching a small unit which suddenly emerged from a hidden place with much dust and noise to make the enemy believe that reinforcements had arrived.

Vegetius (Frontinus) is full of the oddest stratagems to mislead the enemy or demoralize his troops.
Once the enemy cracked, however, they were not to be surrounded, but an easy escape route left open. The reasons for this were that trapped soldiers would fight to the death but if they could get away, they would, and were exposed to the cavalry waiting on the flanks.

This important section of Vegetius closes with the tactics to be used in the case of a withdrawal in the face of the enemy. This highly difficult operation requires great skill and judgement. Both your own men and those of the enemy need to be deceived. It is suggested that your troops be informed that their retirement is to draw the enemy into a trap and the movement can be screened from the enemy with the use of cavalry across the front. Then the units are drawn off in a regular manner, but these tactics can only be employed if the troops have not yet been engaged. During a retreat units are detached and left behind to ambush the enemy if there is a hasty or incautious advance, and in this way tables can often be turned.

On a wider front, the Romans used tactics of denying their opponents the means of sustained warfare. For this they employed the tactic of vastatio. It was in effect the systematic revaging of an enemy's territory. Crops were destroyed or carried off for Roman use, animals were taken away or simply slaughtered, people were massacred or enslaved.
The enemy's lands were decimated, denying his army any form of support. Sometiems these tactics were also used to conduct punitive raids on barbarian tribes which had performed raids across the border.
The reasons for these tactics were simple. In the case of punitive raids they spread terror among the neighbouring tribes and acted as a deterrent to them. In the case of all-out war or the quashing rebels in occupied territories these harsh tactics denied any enemy force the support they needed to sustain a lengthy struggle.