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. Traditionally infantry have made up the largest proportion of an army's fighting strength, and consequently often suffer the heaviest casualties. Throughout history, infantrymen have sought to minimise their losses in both attack and defence through effective tactics.
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.
Modern infantry tactics vary with the type of infantry deployed. Armoured and mechanised infantry are moved and supported in action by vehicles, while others may operate amphibiously from ships, or as airborne troops inserted by helicopter, parachute or glider, whereas light infantry may operate mainly on foot. In recent years, peacekeeping operations in support of humanitarian relief efforts have become particularly important. Tactics also vary with terrain. Tactics in urban areas, jungles, deserts or arctic areas are all markedly different.
The infantry phalanx was a Sumerian tactical formation as far back as the third millennium BC. It was a tightly knit group of hoplites, generally upper and middle class men, typically eight to twelve ranks deep, armored in helmet, breastplate, and greaves, armed with two- to three-meter (6–9 foot) pikes and overlapping round shields. It was most effective in narrow areas, such as Thermopylae, or in large numbers. Although the early Greeks focused on the chariot, because of local geography, the phalanx was well developed in Greece and had superseded most cavalry tactics by the Greco-Persian Wars. In the fourth century BC Philip II of Macedon reorganized his army, with emphasis on phalanges, and the first scientific military research. Theban and Macedonian tactics were variations focused on a concentrated point to break through the enemy phalanx, following the shock of cavalry. Carefully organized—into tetrarchia of 64 men, taxiarchiae of two tetrarchiae, syntagmatae of two taxiarchiae, chilliarchiae of four syntagmatae, and phalanges of four chilliarchiae, with two chilliarchiae of peltasts and one chilliarchia each of psiloi and epihipparchy (cavalry) attached—and thoroughly trained, these proved exceedingly effective in the hands of Alexander III of Macedon.