Maniple (military unit) in the context of Samnite Wars

The Battle of Pydna took place in 168 BC between Rome and Macedon during the Third Macedonian War. The battle saw the further ascendancy of Rome in the Hellenistic world and the end of the Antigonid line of kings, whose power traced back to Alexander the Great. The battle is also considered to be a victory of the Roman legion's manipular system's flexibility over the Macedonian phalanx's rigidity.

The Roman army of the mid-Republic, also called the manipular Roman army or the Polybian army, refers to the armed forces deployed by the mid-Roman Republic, from the end of the Samnite Wars (290 BC) to the end of the Social War (88 BC). The first phase of this army, in its manipular structure (290–c. 130 BC), is described in detail in the Histories of the ancient Greek historian Polybius, writing before 146 BC.

The central feature of the mid-Republican army was the manipular organisation of its battle line. Instead of a single, large mass (the phalanx) as in the Early Roman army, the Romans now drew up in three lines (triplex acies) consisting of small units (maniples) of 120 men, arrayed in chessboard fashion, giving much greater tactical strength and flexibility. This structure was probably introduced in c. 300 BC during the Samnite Wars. Also probably dating from this period was the regular accompaniment of each legion by a non-citizen formation of roughly equal size, the ala, recruited from Rome's Italian allies, or socii. The latter were about 150 autonomous states which were bound by a treaty of perpetual military alliance with Rome. Their sole obligation was to supply to the Roman army, on demand, a number of fully equipped troops up to a specified maximum each year. Evidence from Roman army camps near Numantia in Spain suggests that a much larger tactical unit, the cohort (480 men, equivalent to 4 maniples) already existed, alongside maniples, in the period 153–133 BC. By c. 100 BC, cohorts appear to have fully replaced maniples as the basic tactical unit.

Centuria (Latin: [kɛn̪ˈt̪ʊria]; pl.: centuriae) is a Latin term (from the stem centum meaning one hundred) denoting military units originally consisting of 100 men. The size of the centuria changed over time, and from the first century BC through most of the imperial era the standard size of a centuria was 80 soldiers. By the time of the Roman Empire, ordo became a synonym of centuria (in 4 BC it was used for a maniple). Ten contuberniums formed a century, composed of 80 legionaries. Commanding this unit was the centurion, a veteran expert in combat analogous to an non-commissioned officer. His role was not only that of a leader, but also that of an instructor and responsible for discipline within his unit. The centurion was assisted by the optio, his second-in-command and other officers such as the tesserarius, in charge of security, the signifer, who carried the banner, and the cornicen, who transmitted orders by trumpet.

The primus pilus (lit. "first maniple of triarii") or primipilus was the senior centurion of the first cohort in a Roman legion, a formation of five double-strength centuries of 160 men each; he was a career soldier and advisor to the legate. The primus pilus would remain in command for one year. They could continue to serve in the army after their term ended if there was a vacancy in command or if they wished to become an independent commander of an auxilia unit or the praefectus castrorum.

During the Roman Empire, the emperor Claudius created the office of primus pilus iterum. To become the primus pilus iterum an officer must have formerly served as a tribune in the vigiles, cohortes urbanae, or Praetorian Guard. The primus pilus iterum would hold the responsibility of a praefectus castrorum but with higher pay.

A cohort (from the Latin cohors, pl.: cohortes; see wikt:cohors for full inflection table) was a standard tactical military unit of a Roman legion. Although the standard size changed with time and situation, it was generally composed of 480 soldiers. A cohort is considered to be the equivalent of a modern military battalion. The cohort was the most important tactical unit in the Roman army. They could operate independently and offered flexibility and maneuverability, allowing the legions to quickly adapt to different combat situations. The cohort replaced the maniple. From the late second century BC and until the middle of the third century AD, ten cohorts (about 5,000 men total) made up a legion. Cohorts were named "first cohort", "second cohort", etc. The first cohort consisted of experienced legionaries, while the legionaries in the tenth cohort were less experienced.

The scutum (Classical Latin: [ˈskuːt̪ʊ̃]; pl.:scuta) was a type of shield used among Italic peoples in antiquity, most notably by the army of ancient Rome starting about the fourth century BC.

The Romans adopted it when they switched from the military formation of the hoplite phalanx of the Greeks to the formation with maniples (Latin: manipuli). In the former, the soldiers carried a round shield, which the Romans called a clipeus. In the latter, they used the scutum, which was larger. Originally, it was oblong and convex, but by the first century BC, it had developed into the rectangular, semi-cylindrical shield that is popularly associated with the scutum in modern times. This was not the only kind the Romans used; Roman shields were of varying types depending on the role of the soldier who carried it. Oval, circular and rectangular shapes were used throughout Roman history.

Hastati (sg.: hastatus) were a class of infantry employed in the armies of the early Roman Republic, who originally fought as spearmen and later as swordsmen. These soldiers were the staple unit after Rome threw off Etruscan rule. They were originally some of the poorest men in the legion, and could afford only modest equipment—light chainmail and other miscellaneous equipment. The Senate supplied their soldiers with only a short stabbing sword, the gladius, and their distinctive squared shield, the scutum. The hastatus was typically equipped with these, and one or two soft iron tipped throwing spears called pila. This doubled their effectiveness, not only as a strong leading edge to their maniple, but also as a stand-alone missile troop. Later, the hastati contained the younger men rather than just the poorer, though most men of their age were relatively poor. Their usual position was the first battle line. They fought in a quincunx formation, supported by lighter infantry. The enemy was allowed to penetrate the first battle line consisting of hastati, after which the enemy would deal with the more hardened, seasoned soldiers, the principes. They were eventually disbanded after the so-called "Marian reforms" of 107 BC.

Triarii (sg.: triarius) ("the third liners") were one of the elements of the early Roman military manipular legions of the early Roman Republic (509 BC – 107 BC). They were the oldest and among the wealthiest men in the army and could afford high quality equipment. They wore heavy metal armor and carried large shields, their usual position being the third battle line. They were equipped with spears and were considered to be elite soldiers among the legion.

During the Camillan era, they fought in a shallow phalanx formation, supported by light troops. In most battles triarii were not used because the lighter troops usually defeated the enemy before the triarii were committed to the battle. They were meant to be used as a decisive force in the battle, thus prompting an old Roman saying: res ad triarios venit, 'it comes down to the triarii', which meant carrying on to the bitter end.

Velites (Latin: [ˈweːlɪteːs]; sg.: veles) were a class of infantry in the Roman army of the mid-Republic from 211 to 107 BC. Velites were light infantry and skirmishers armed with javelins (Latin: hastae velitares), each with a 75cm (30 inch) wooden shaft the diameter of a finger, with a 25cm (10 inch) narrow metal point, to fling at the enemy. They also carried short thrusting swords, or gladii, for use in melee. They rarely wore armour as they were the youngest and poorest soldiers in the legion and could not afford much equipment, but they did carry small wooden shields called parma for protection. The velites were placed at the front partly for tactical reasons, and also so that they had the opportunity to secure glory for themselves in single combat.

Velites did not form their own units; a number of them were attached to each maniple of hastati, principes and triarii. They were typically used as a screening force, driving off enemy skirmishers and disrupting enemy formations with javelin throws before retiring behind the lines to allow the heavier-armed hastati to attack. They were normally the ones who engaged war elephants and chariots if they were present on the field, such as in the Battle of Zama, in 202 BC. Their high mobility and ranged weaponry made them much more effective against these enemies than heavy infantry. An early Roman legion contained approximately 1000 velites. Velites were disbanded after the so-called "Marian reforms". Lucilius suggests that rorarii and velites were interchangeable, with velites gradually superseding rorarii. Another theory is that the leves' equipment was upgraded until they were at the same level as the rorarii, and they both collectively became known as the velites.