The earliest Roman army formation was the phalanx, the formation used by the Greeks, Macedonians, and Carthaginians. For the Romans the phalanx proved to be too unwieldy a unit to fight on hilly and broken ground and they soon began to change the nature of their battle formations. The result was the legion. Unlike the phalanx, the legion was not a static form; it varied greatly over the centuries.

The term legion did not originally mean any specific type of military formation. Its origin probably denoted those who were chosen for military service during the annual public assembly of citizens. As it developed, the legion became a unit of from 4,000 to 6,000 heavy infantry supported by cavalry and light infantry. The term infantry simply means soldiers who fight on foot; the terms light and heavy refer to the kinds and weight of their weapons.