Though its exact origins are unknown, the Roman legion seems to have developed from the phalanx. In fact, it was a collection of small, well-integrated, well-coordinated phalanxes arrayed in checkerboard formation and operating as a team. Hellenistic heavy infantry relied on the pike almost exclusively; the legion, by contrast, possessed both shock and firepower—the former in the form of the short sword, or gladius, the latter delivered by the javelin, or pilum, of which most (after 100 bc, all) legionnaires carried two. Screening was provided by light troops moving in front, cohesion by pikemen in the third and rearmost rank. Short arms made it easier for individual soldiers or subunits to turn and change direction. Too, careful articulation, a well-rehearsed command system, and the use of standards—which do not seem to have been carried by Hellenistic armies—made the legion a much more flexible organization than the phalanx. No Greek army could have imitated the movement carried out by Caesar’s troops at Ruspinum in Africa in 47 bc, when part of a legion was made to turn around and face an enemy cavalry force coming from the rear. As numerous battles showed, where the terrain was uneven and the chain of command broke down, the legion’s advantage was even more pronounced. A phalanx whose ranks were thrown into disorder and penetrated by the enemy’s infantrymen was usually lost; a legionary commander could rely on his soldiers’ swords to deal with intruders, meanwhile bringing up additional units from both flanks.

As a formation whose main power consisted of its heavy infantry, the legion remained unmatched until the introduction of firearms and beyond. Attempts to imitate its armament and methods were made right down to the 16th century, and even today some countries still call their forces legions in commemoration of its prowess. During the 1st century bc, legionary organization underwent some changes at the hands of Gaius Marius and Lucius Cornelius Sulla until it reached the zenith of its development about the time of Caesar. Subunits became larger, and the legion incorporated a detachment of heavy cavalry as well as field artillery in the form of catapults—thus turning into a combined-arms unit and becoming a true forerunner of the modern division. Yet the legion, too, had its limitations when it came to fighting in the dense forests of Germany or, even more so, the open deserts of the Middle East. As Marcus Licinius Crassus’ disastrous defeat at Carrhae in 53 bc demonstrated, it met its match in the eastern light cavalry, with which it could never really come to grips, so that, even after repeated attempts, the Romans failed to subdue Parthia as they had so many other countries. The lesson was not lost. From the time of Belisarius in the 6th century ad, the Byzantine army always supplemented its infantry and heavy cavalry with units of horse archers, usually consisting of mercenaries recruited from various barbarian tribes. In this way, they were able to counter the Arabs and, later, the Seljuqs.