The early Roman had to spend much time fighting in defence of the fields he tilled. Every property owner was expected to fight for his city, which had no professional army, only a citizen militia, called up as need demanded. The primitive organization, based on Thousands, soon proved inadequate for military purposes, and by the Servian reform the community was organized as an army and the ‘nation in arms’ became the chief political assembly. The complex question of the development of the early army has already been discussed (pp. 64ff. and relevant notes), and we have seen how military, political, economic and social needs led to the introduction of a battle-line of heavily armed infantry (hoplites). This new levy (Legio) was at some point divided into two groups or legions, though it is uncertain whether this occurred at the time of the fall of the monarchy (to provide each consul with a command) or in the difficult days after the Gallic invasion. During the siege of Veii (i.e. c. 400 BC) pay was introduced in order to enable the yeoman soldiers to keep the field throughout the year: the first step towards turning a citizen militia into a professional army had been taken. Then, as Rome’s territory and population increased, it was found necessary in the Second Samnite War to levy two consular armies of two legions each.
The introduction of pay meant that each man could now arm himself, and it was not necessary to place the wealthier men in the first line. Gradually experience, skill and age supplanted wealth and the census as the basis of military arrangement. The heavy infantry was drawn up in three lines, hastati, principes and triarii, though the light-armed troops were still recruited on a strict census basis from the two lowest classes. All three lines had practically the same armour: helmets, breastplates, shields, greaves, javelins and swords. The front line, despite its name, did not consist of spearmen (hastati), and the second line (principes) had ceased to form the front of the battle array; the triarii were a third supporting line of veterans, with slightly lighter armour. This important reorganization was accompanied by a fundamental change in tactics: the phalanx gave place to the manipular system which endured till the last century of the Republic. The legion was divided into 30 maniples, each comprising 120 men. Intervals were left between the maniples in each line, and the maniples of the second line covered the spaces of the first. The object of the reform was to give greater elasticity than the mass-tactics of the phalanx. The army was thus divided horizontally into three lines and vertically into maniples. A corresponding change was made in equipment; the most important introduction was the pilum, a javelin 6½ feet in length, half wood and half iron, which replaced the spear (hasta) in the two front lines. The date of these changes is disputed. Some assign them to the years after the Gallic invasion and to the wisdom of Camillus. Others suppose that the flexible manipular formation was forced upon the Romans when operating in the rough hill country of Samnium; also that the pilum derived from Samnium.4Further, contact with the gaily-dressed cavaliers of Campania and Samnium may have helped to transform the mounted infantry of Rome into real cavalry. But Roman cavalry was always weak, so that the Romans depended more on their allies; and even Italian cavalry from the time of the Hannibalic War was supplemented by hiring foreign auxiliaries. The allies sent forces to the Roman armies; the maximum levy of each state was fixed by its treaty, though normally the whole contingent would not be called out. The allied troops served with the legions under Roman officers (praefecti sociorum).
Such in brief was the army that conquered Italy; but before it conquered the Mediterranean world it was reformed still further. Manipular tactics had ousted the rigid phalanx, but the battle-line was not sufficiently flexible; it still relied on mere push and weight and could not wheel or turn with any ease. When faced by a mobile enemy, the Roman legions collapsed. Cannae showed with tragic clearness the vulnerability of the legions, which broke because they could not bend, and the inadequacy of Roman cavalry; once the wings had been swept clear of the covering cavalry the infantry was outflanked and surrounded before it could break through the enemy’s centre. This weakness was gradually rectified by the genius of Scipio Africanus, the first Roman general as such, who went to Spain, not as consul or praetor, but as an army commander with proconsular power, to campaign not for a year but until victory was won. There he built up his new model army. He threw over the close maintenance of the triple line and created lines which operated independently. This more flexible weapon was used with increasing skill, at Baecula, Ilipa and Campi Magni, until it crushed Hannibal at Zama. Scipio also made the individual more effective when he had broken away from the compact mass by adequate training in arms drill. The Spanish sword with its well-tempered point was adopted, and the pilum may have been improved from Spanish models. Rome’s lack of cavalry was counteracted by alliance with the native princes of Spain and Numidia. But Scipio’s greatest tactical contribution was that he made the units more self-reliant and thus led on to the next development of Roman tactics when the cohort, a grouping together of three maniples, became the unit; possibly some experimental tactical use of cohorts already may have been made by Scipio himself.
The Hannibalic War tended to professionalize the Roman army, but it was followed by a reaction to the more amateur methods of earlier days and many men were glad to return to civil life. Despite easier conditions of service and under some commanders a dangerously lax discipline, there was a marked reluctance for military service on the part of some. But others had gained an appetite for soldiering and did not wish to settle back on the land. The wars of conquest and long periods of service offered such men a profession, though a precarious one, as there was no standing army: forces were still raised from the propertied classes as need demanded and demobilized when operations were over.5 Conditions of life are well illustrated by a speech which Livy puts into the mouth of a veteran volunteer in 171 BC: ‘I am Spurius Ligustinus, and I come from the Sabine country. My father left me less than an acre of land and a small cottage in which I was born and bred; I live there now… I have six sons and two daughters, both now married… I became a soldier in the consulship of P. Sulpicius and C. Aurelius [200 BC]… I served in the ranks for two years against King Philip. In the third year as a reward for bravery T. Quinctius Flamininus made me a junior centurion. Discharged after Philip’s defeat, I at once went to Spain as a volunteer with M. Porcius  who thought me worthy of promotion to a higher rank among the centurions. A third time I again enlisted as a volunteer in the army which was sent against the Aetolians and King Antiochus; I was promoted by M’. Acilius … We were brought back to Italy and then I served in two legions which were raised for a year. Then I served in Spain, once under Q. Fulvius Flaccus  and again under Ti. Sempronius Gracchus. Four times within a few years I have been first centurion of a legion; I have been rewarded thirty-two times for bravery by my generals; I have received six civic crowns. I have served in twenty-two annual campaigns, and I am over fifty years old’ (Livy, xlii, 34).
During the Republic there was no standing navy; ships were built and fitted out as required. Before the First Punic War the navy was small and a few ships were supplied by the Italiot towns; but the war forced Rome to become a great sea power. In their amateur but successful manner the Romans put one or both of the consuls in command of the fleet. During part of the Hannibalic War the fleet sometimes formed the separate ‘province’ of a praetor; sometimes it was commanded by a consul or his deputy (praefectus). The oarsmen and sailors were supplied by the allies and maritime colonies, and after 217 by libertini also. The marine troops were usually drawn from the Roman proletariat, the sixth class, or occasionally from allies and Latins. Naval service was not popular. The standard vessel was the quinquereme; the trireme and quadrireme were also used, as well as lighter craft such as the Illyrian lembi. As the admiral’s flagship one of the great Hellenistic ships might be used. Although the Roman navy played an important role in her conquest of the Mediterranean world, it did not adequately police the seas in times of peace, so that piracy became a real danger to Italian shipping.