Formations of the Legion
The entire foundation of Roman infantry tactics was the idea that by keeping troops in order, one could fight more effectively. Most military commanders of the day simply had their troops rush wildly at the enemy, relying on superior numbers, better soldiers, or luck to carry the day. The Romans realized that they could not always rely on these, so they turned to strategy. Each situation was handled differently, taking into account terrain, the type and strength of the opponent's troops, and the type and strength of the Roman's troops. Here are some common formations, and tactics that were organized by formations.
This was the default arrangement for a full legion in battle. The cavalry rode up front, on the sides where they could protect the flanks. In between them were two rows of five cohorts. The rightmost cohort consisted of ~1100 infantry and ~30 mounted troops, while the others contained ~550 infantry and ~65 cavalry. Behind the main group were seven units of light troops, followed by seven units of reserves.
When the legion was in transit, a very different arrangement was required. The main part of the cavalry rode up front as a vanguard, followed by the infantry, in a long column of cohorts. Behind them came the army's baggage, servants, and vehicles, guarded by several units of cavalry. At the end came the best units of both infantry and cavalry, to defend against attacks from the rear. The lighter units were arranged around the edges to act as scouts.
"A general whose troops are superior in number and bravery should engage in the oblong square, which is the first formation."
The First Formation
This tactic, designed for level terrain, assumes that your wings are more powerful. Should the enemy make their way around your flanks, the reserves will be able to counter. Once their wings are vanquished, you may press the center.
"He who judges himself inferior should advance his right wing against his enemy's left. This is the second formation."
This formation, considered by some to be the best, took advantage of the fact that the left side of a soldier, and so the left side of the army was considered to be weaker, because it had to support the weight of the shield. The right wing moved around the opponent's left, and attacked from the rear. The left wing kept its distance, while the reserves supported the left wing or guarded against the enemy attacking the center.
"If your left wing is strongest, you must attack the enemy's right, according to the third formation."
The third formation was considered something of a desperation move, to be used only when your left wing, usually the weaker side, was stronger than your right. In this attack, the left wing, supplemented by the Roman's best cavalry, attacked the opponent's right wing, while their own right stayed back in relative safety.
"The general who can depend on the discipline of his men should begin the engagement by attacking both of the enemy's wings at once, the fourth formation."
The fourth formation's main advantage was its shock value. The entire army was brought close to the enemy, whereupon both wings charged at the enemy. This would often surprise the opponent, allowing for a quick resolution. However, the attack split the army into three parts, so if the enemy survived the attack, the center of the Roman's forces was vulnerable, and the wings could be fought separately.
"He whose light infantry is good should cover his center by forming them in its front and charge both the enemy's wings at once. This is the fifth formation."
This was a variation of the fourth formation. Light infantry and archers were placed in front of the center, making it far less vulnerable.
"He who cannot depend on either the number or courage of his troops, if obliged to engage, should begin the action with his right and endeavor to break the enemy's left, the rest of his army remaining formed in line perpendicular to the front and extended to the rear like a javelin. This is the sixth formation."
The sixth formation was similar to the second, with both having the right wing attacking the opponent's left from behind. In this attack, the enemy's left wing cannot be reinforced, for fear that it would leave an opening for the Romans to exploit.
"If your forces are few and weak in comparison to the enemy, you must make use of the seventh formation and cover one of your flanks either with an eminence, a city, the sea, a river, or some protection of that kind."
When the Romans were outnumbered or had inferior troops, this was often the only hope for victory. The left flank was kept guarded by whatever protection was available. The right was protected by the light troops and cavalry. With both sides well covered, the army had little to fear from an attack.