Phalanx formation in the context of "Nicanor (son of Parmenion)"

The Seleucid army was the army of the Seleucid Empire, one of the numerous Hellenistic states that emerged after the death of Alexander the Great.

As with the other major Hellenistic armies, the Seleucid army fought primarily in the Greco-Macedonian style, with its main body being the phalanx. The phalanx was a large, dense formation of men armed with small shields and a long pike called the sarissa. This form of fighting had been developed by the Macedonian army in the reign of Philip II of Macedon and his son Alexander the Great. Alongside the phalanx, the Seleucid armies used a great deal of native and mercenary troops to supplement their Greek forces, which were limited due to the distance from the Seleucid rulers' Macedonian homeland.

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.

The line formation is a standard tactical formation which was used in early modern warfare.It continued the phalanx formation or shield wall of infantry armed with polearms in use during antiquity and the Middle Ages.

The line formation provided the best frontage for volley fire, while sacrificing maneuverability and defence against cavalry. It came to the fore during the Age of Reason, when it was used to great effect by Frederick the Great and his enemies during the Seven Years' War. The line formation was very successfully first used with combined arms in the Thirty Years War by the Swedish king Gustavus Adolphus the Great, at the Battle of Breitenfeld.

Infantry tactics are the combination of military concepts and methods used by infantry to achieve tactical objectives during combat. The role of the infantry on the battlefield is, typically, to close with and engage the enemy, and hold territorial objectives; infantry tactics are the means by which this is achieved. Infantry commonly makes up the largest proportion of an army's fighting strength, and consequently often suffers the heaviest casualties. Throughout history, infantrymen have sought to minimise their losses in both attack and defence through effective tactics. (For a wider view of battle and theater tactics see: Military strategy)

Infantry tactics are the oldest method of warfare and span all eras. In different periods, the prevailing technology of the day has had an important impact on infantry tactics. In the opposite direction, tactical methods can encourage the development of particular technologies. Similarly, as weapons and tactics evolve, so do the tactical formations employed, such as the Greek phalanx, the Spanish tercio, the Napoleonic column, or the British 'thin red line'. In different periods the numbers of troops deployed as a single unit can also vary widely, from thousands to a few dozen.