The legion was commanded in the earlier times in turns by its six military tribunes; each of them held the command for two months. After the first civil war, legates were placed as standing chiefs at the head of every legion; the tribunes now were mostly officers intrusted with the staff or administrative business. The difference of arma-men^ of the three lines had disappeared before the time of Marius. The pilum had been given to all three lines of the legion; it was now the national arm of the Romans. The qualitative distinction between the three lines, as far as it was based upon age and length of service, soon disappeared too. In the battle of Metellus against Jugurtha, there appeared, according to Sallust, for the last time hastati, principes, and triarii. Marius now formed out of the 30 manipuli of the legion 10 cohorts, and disposed them in two lines of five cohorts each. At the same time, the normal strength of the cohort was raised to (500 men; the first cohort, under the primus pilus, carried the legionary eagle. The cavalry remained formed in turmœ of 30 rank and file and three decurions, the first of whom commanded the turma.

The armature of the Roman infantry consisted of a shield of semi-cylindric shape, 4 feet by 2 1/2, made of wood, covered with leather and strengthened with iron fastenings; in the middle it had a boss (umbo) to parry off spear thrusts. The helmet was of brass, generally with a prolongation behind to protect the neck, and fastened on with leather bands covered with brass scales. The breastplate, about a foot square, was fastened on a leather corslet with Scaled straps passing over the shoulder; for the centurions, it consisted of a coat of mail covered with brass scales. The right leg, exposed when advanced for the sword thrust, was protected by a brass plate. Besides the short sword, which was used for thrusting more than for cutting, the soldier carried the pilum, a heavy spear 4 1/2 feet wood with a projecting iron point of 1 1/2 foot, or nearly 6 feet in all long, but 2 1/2 inches square in the wood, and weighing about 10 or 11 lbs. When thrown at 10 or 15 paces distance, it often penetrated shield and breastplate, and almost always threw down its man. The velites, lightly equipped, carried light short javelins. In the later periods of the republic, when bar-baric auxiliaries undertook the light service, this class of troops disappeared entirely.

The cavalry were provided with defensive armor similar to that of the infantry, a lance, and a longer sword. But the Roman national cavalry was not very good, and preferred to fight dismounted. In later periods Numidian, Spanish, Gallic, and German horsemen supplanted it. The tactical disposition of the troops admitted of great mobility. The manipuli were formed at intervals equal to their extent of front; the depth varied from 5 or 6 to 10 men. The manipuli of the second line were placed behind the intervals of the first; the triarii still further to the rear, but in one unbroken line. According to circumstances, the manipuli of each line could close up or form line without intervals, or those of the second line could march up to fill the intervals of the first; or else, where greater depth was required, the manipuli of the principes closed up each in rear of the corresponding manipulus of the hastati, doubling its depth. When opposed to the elephants of Pyrrhus, the three lines all formed with intervals, each manipulus covering the one in its front, so as to leave room for the animals to pass straight through the order of battle. In this formation the clumsiness of the phalanx was in every way successfully overcome.

The legion could move and manoeuvre, without breaking its order of battle, in ground where the phalanx durst not venture without the utmost risk. One or two manipuli at most would have to shorten their front to defile past an obstacle; in a few moments the front was restored. The legion could cover the whole of its front by light troops, as they could retire, on the advance of the line, through the intervals. But the principal advantage was the disposition in a plurality of lines, brought into action successively, according to the requirements of the moment. With the phalanx, one shock had to decide. No fresh troops were in reserve to take up the fight in case of a reverse; in fact, that case was never provided for. The legion could engage the enemy with its light troops and cavalry on the whole of his front; could oppose to the advance of his phalanx its first line of hastati, which was not so easily beaten, as at least six of the ten manipuli had first to be broken singly; could wear out the strength of the enemy by the advance of the hastati, and finally decide the victory by the triarii.

Thus the troops and the progress of the battle remained in the hands of the general, while the phalanx, once engaged, was irretrievably engaged, with all its strength, and had to see the battle out. If the Roman general desired to break off the combat, the legionary organization permitted him to take up a position with his reserves, while the troops engaged before retired through the intervals, and took up a position in their turn. Under all circumstances, there was always a portion of the troops in good order, for even if the triarii were repulsed, the two first lines bad reformed behind them. When the legions of Flamininus met Philip's phalanx in the plains of Thessaly, their first attack was at once repulsed; but charge following charge, the Macedonians got tired and lost part of their compactness of formation; and wherever a sign of disorder manifested itself, there was a Roman manipulus to attempt an inroad into the clumsy mass. At last, 20 manipuli attacking the flanks and rear of the phalanx,, tactical continuity could no longer be maintained; the deep line dissolved into a swarm of fugitives, and the battle was lost. Against cavalry, the legion formed the orb is, a sort of square with baggage in the centre.