When the three separate and successive maniples of the first cohort were united in order to form the united battle cohort of Marius and of Caesar, the same brain placed the most reliable men in the last lines, i.e., the oldest. The youngest, the most impetuous, were in the first lines. The legion was not increased simply to make numbers or mass. Each had his turn in action, each man in his maniple, each maniple in its cohort, and, when the unit became a cohort, each cohort in the order of battle.
We have seen that the Roman theory dictated a depth of ranks to furnish successive lines of combatants. The genius of the general modified these established formations. If the men were inured to war, well-trained, reliable, tenacious, quick to relieve their file leaders, full of confidence in their general and their own comrades, the general diminished the depth of the files, did away with the lines even, in order to increase the number of immediate combatants by increasing the front. His men having a moral, and sometimes also a physical endurance superior to that of the adversary, the general knew that the last ranks of the latter would not, under pressure, hold sufficiently to relieve the first lines nor to forbid the relief of his own. Hannibal had a part of his infantry, the Africans, armed and drilled in the Roman way; his Spanish infantrymen had the long wind of the Spaniards of to-day; his Gallic soldiers, tried out by hardship, were in the same way fit for long efforts. Hannibal, strong with the confidence with which he inspired his people, drew up a line less deep by half than the Roman army and at Cannae hemmed in an army which had twice his number and exterminated it. Caesar at Pharsalus, for similar reasons, did not hesitate to decrease his depth. He faced double his strength in the army of Pompey, a Roman army like his own, and crushed it.
We have mentioned Cannae and Pharsalus, we shall study in them the mechanism and the morale of ancient combat, two things which cannot be separated. We cannot find better examples of battle more clearly and more impartially exhibited. This is due in one case to the clear presentation of Polybius, who obtained his information from the fugitives from Cannae, possibly even from some of the conquerors; in the other it is due to the impassive clearness of Caesar in describing the art of war.