Changes

The army’s shifting mission was one reason for its transformation. In the early first century bc, the army had been involved in expanding Rome’s territorial boundaries under generals Pompey the Great and Julius Caesar. This would give way to the modified posture of maintaining these boundaries in the first to early second centuries ad. This posture was not rigid, however, since emperors Claudius and Trajan launched campaigns in Britannia (ad 43) and Dacia (ad 102, 104) with the intent of expansion. The emperor Vespasian and his son, Titus, conquered Judaea (ad 66–73) to put down a Jewish revolt. Nevertheless, during this period, the army was downsized by half (from roughly 60 to 28 legions) and made into a professional force; the Romans would no longer fight wars with non-career citizen-soldiers.

Changing battlefield tactics also provided an impetus for the army to change. The invasion of Celtic tribes (Gauls) in the fourth century bc and the collapse of the army on the battlefield at Allia forced a shift in tactics. The Roman army had been deployed in a traditional Greek-style phalanx formation—hoplite soldiers in a tight formation with overlapping shields and spears pointed outward. This evolved to a looser, more flexible presentation on the battlefield. This would evolve after the reforms of Gaius Marius (157–86 bc), who streamlined the administration of the army and instituted uniformity in terms of equipment and training in the wake of three consecutive defeats of the army at the hands of Celtic tribes (109, 107, and 105 bc). Marius’ most significant reform, however, was changing the battlefield presentation of the army to a cohort legion.