The Roman army was under command of an imperator, here Caesar. The second highest officer was the quaestor, who served as chief-of-staff and quartermaster general; this position in the Gallic War is filled by Marcus Licinius Crassus. The next rank of officers were the legati, men of senatorial rank who were often used as legion commanders by Caesar. Caesar was entitled to ten legati even when he had fewer legions in action. In the period covered by the Gallic Wars, we read about fourteen men named as legati: Marcus Antonius, Lucius Caesar, Lucius Aurunculeius Cotta, Gaius Fabius, Titus Labienus, Quintus Pedius, Lucius Munatius Plancus, Gaius Caninus Rebilus, Gaius Antistius Reginus, Publius Sulpicius Rufus, Quintus Titurius Sabinus, Titus Sextius, Marcus Silanus, and Gaius Trebonius. In addition, there were several officers placed in command of a legion or several legions, men who were not cited by rank but seem to have been fulfilling the role of legatus. A few of these are Cicero, Publius Crassus, and Servius Galba.
Earlier the Roman legion was composed of 6,000 men, but by Caesar's time it seems to have become considerably smaller. The maximum in the Gallic Wars seems to be about 5,000. Sometimes the legions are even smaller — at one point we read that two legions and their accompanying cavalry total only 7,000 men.
In command of each legion, usually, was a legatus. The legion was composed of ten cohorts, each commanded by tribuni militum, military tribunes. Each cohort was composed of six units of 100 men each, each of which was commanded by one of the ordines, centurions. Accompanying each legion, of course, were forces of cavalry and auxiliary foot soldiers. These units were recruited in the field and were commanded by officers titled praefecti.