The new tactical fighting system the Romans devised after their disastrous defeat by the Gauls in the early fourth century B. C. broke down the old phalanx into smaller units—the maniples. On the battlefield, the maniples were arranged into lines with open spaces between both the maniples and the lines. This allowed individual units to move back and forth with ease, permitting tired troops to fall back and rest while fresh ones pressed forward into the fray. When need dictated, various-sized contingents of maniples could also separate from the army's main body and fight on their own.

Because the new system was both flexible and effective, it remained in place for a long time. Even after the army's structure underwent another important revision in Marius's time (the late second century to early first century B. C...