Caesar's legionary was no longer a citizen-soldier, as in the Punic wars; he was a professional, or a mercenary. He served for a livelihood, not as a duty. The legion was no longer set up in three lines according to property rating; it was marshaled in two or three lines of cohorts, the cohort being a body of four to six hundred men, ranked according to military qualities, and ten cohorts went to the legion. The men retained substantially the old equipment; they occupied in line a space of but three feet front instead of five. The intervals between cohorts had sensibly decreased. The camp and camp-followers, musicians, standards and petty details of all kinds remained much as before. Light troops and cavalry were recruited from conquered tribes. Each legion had six tribunes who commanded it in turn under a legate. The general staff of the army had quartermasters, aides, engineers, lictors, scouts and a body-guard. The legionary's pay was about that of a day-laborer, but largesses and booty were bountiful. For defense, the legion or army formed square or circle. It readily employed into column or deployed into line. The orders of march were accurately laid down and well observed. The average march was about fifteen miles. The train was less long than ours, there being neither artillery nor ammunition. For battle the army was drawn up on the slope of a hill; it still attacked, as had always been its habit. The legion of the Punic war was good because the men were good; Caesar's was effective because he was able.
The tactical formation of the early Roman army was described in the volume on Hannibal. Considerable changes in this tactical formation had taken place since the Punic wars; some were introduced by Marius, or by Caesar as a consequence of his campaigns. The Roman soldier as to arms, equipment and minor tactics, was to all intents and purposes the same as he had been in the time of Hannibal. He still wore helmet, cuirass, and greave on right leg; he still bore spear, shield and sword. But in character, quality and discipline, he was no longer the splendid citizen-soldier of that day. He was a professional if a Roman; a mercenary if a foreigner; and as such he was exactly what his commander made him. A similar change had obtained in the officers. The six tribunes of a legion were no longer appointed for their military qualifications or long service, but claimed their rank on the score of political or social standing, or of friendship for the chief.
The original smaller tactical unit of the Romans had been the century. It next became the maniple of two centuries. Later the three maniples of the hastati, principes and triarii, with some cavalry and velites, were merged into one body called a cohort. But the name alone remained. Under Caesar the cohort was no longer the ancient one, but a body the evolution of which has already been traced. It was divided only for the smaller details. It was practically the tactical unit of the legion, and all maneuvers were by cohorts. The three-class formation of each cohort had entirely disappeared.
It seems odd, with all that has been written about Roman tactics and organization from Polybius to Vegetius and Onosander, that the exact structure of Caesar's legion, cohort, maniple and century cannot be given. The addition of a few words in some paragraphs would elucidate the difficulties we encounter in construing these authors. But, as the omission of the name of some single well-known spot by Polybius has given rise to endless discussion as to Hannibal's route across the Alps, so a certain lapse in explanation in all the Latin authors who treat of tactics has produced many different views as to the details of organization of Caesar's legion.