Upon the history of no people has the character of its military institutions exercised a more profound effect than upon that of Rome. The Roman military system rested upon the universal obligation of the male citizens to render military service, but the degree to which this obligation was enforced varied greatly at different periods. For the mobilization of the man power of the state was dependent upon the type of equipment, methods of fighting, and organization of tactical units in vogue at various times, as well as upon the ability of the state to equip its troops and the strength of the martial spirit of the people. *The army of the primitive state.* In all probability the earliest Roman army was one of the Homeric type, where the nobles who went to the battlefield on horseback or in chariots were the decisive factor and the common folk counted for little. *The phalanx organization.* However, at an early date, under Etruscan influences according to tradition, the Romans adopted the phalanx organization, making their tactical unit the long deep line of infantry armed with lance and shield. Those who were able to provide themselves with the armor necessary for taking their place in the phalanx formed the _classis_ or “levy.” The rest were said to be _infra classem_, and were only called upon to act as light troops. But military necessities compelled the state to incorporate with the heavy-armed infantry increasingly large contingents of the less wealthy citizens, who could not provide themselves with the full equipment of those in the _classis_, but who could form the rear ranks of the phalanx. As a result of this step the citizens were ultimately divided into five orders or classes on the basis of their property, and probably in raising the levy the required number of soldiers of each class was drafted in equal proportions from the several tribes. The first three classes constituted the phalanx, while the fourth and fifth continued to serve as light troops (_rorarii_). Those who lacked the property qualification of the lowest class were only called into service in cases of great emergency. For such a system the taking of an accurate census was essential, and it is more than likely that the office of censor was instituted for this purpose. As we have seen, it was from this organization of the people for military purposes that there developed the Assembly of the Centuries. The introduction of pay for the troops in the field at the time of the siege of Veii both lessened the economic burden which service entailed upon the poorer soldiers and enabled the Romans to undertake campaigns of longer duration, even such as involved winter operations.