History

The first warriors, adopting hunting weapons or improvised melee weapons, before the existence of any organised military, likely started essentially as loose groups without any organisation or formation. But this changed sometime before recorded history; the first ancient empires (2500–1500 BC) are shown to have some soldiers with standardised military equipment, and the training and discipline required for battlefield formations and manoeuvres: regular infantry. Though the main force of the army, these forces were usually kept small due to their cost of training and upkeep, and might be supplemented by local short-term mass-conscript forces using the older irregular infantry weapons and tactics; this remained a common practice almost up to modern times.

Before the adoption of the chariot to create the first mobile fighting forces c. 2000 BC, all armies were pure infantry. Even after, with a few exceptions like the Mongol Empire, infantry has been the largest component of most armies in history.

In the Western world, from Classical Antiquity through the Middle Ages (c. 8th century BC to 15th century AD), infantry are categorised as either heavy infantry or light infantry. Heavy infantry, such as Greek hoplites, Macedonian phalangites, and Roman legionaries, specialised in dense, solid formations driving into the main enemy lines, using weight of numbers to achieve a decisive victory, and were usually equipped with heavier weapons and armour to fit their role. Light infantry, such as Greek peltasts, Balearic slingers, and Roman velites, using open formations and greater manoeuvrability, took on most other combat roles: scouting, screening the army on the march, skirmishing to delay, disrupt, or weaken the enemy to prepare for the main forces' battlefield attack, protecting them from flanking manoeuvers, and then afterwards either pursuing the fleeing enemy or covering their army's retreat.

After the fall of Rome, the quality of heavy infantry declined, and warfare was dominated by heavy cavalry, such as knights, forming small elite units for decisive shock combat, supported by peasant infantry militias and assorted light infantry from the lower classes. Towards the end of Middle Ages, this began to change, where more professional and better trained light infantry could be effective against knights, such as the English longbowmen in the Hundred Years' War. By the start of the Renaissance, the infantry began to return to dominance, with Swiss pikemen and German Landsknechts filling the role of heavy infantry again, using dense formations of pikes to drive off any cavalry.

Dense formations are vulnerable to ranged weapons. Technological developments allowed the raising of large numbers of light infantry units armed with ranged weapons, without the years of training expected for traditional high-skilled archers and slingers. This started slowly, first with crossbowmen, then hand cannoneers and arquebusiers, each with increasing effectiveness, marking the beginning of early modern warfare, when firearms rendered the use of heavy infantry obsolete. The introduction of musketeers using bayonets in the mid 17th century began replacement of the pike with the infantry square replacing the pike square.

To maximise their firepower, musketeer infantry were trained to fight in wide lines facing the enemy, creating line infantry. These fulfilled the central battlefield role of earlier heavy infantry, using ranged weapons instead of melee weapons. To support these lines, smaller infantry formations using dispersed skirmish lines were created, called light infantry, fulfilling the same multiple roles as earlier light infantry. Their arms were no lighter than line infantry; they were distinguished by their skirmish formation and flexible tactics.

The modern rifleman infantry became the primary force for taking and holding ground on battlefields worldwide, a vital element of combined arms combat. As firepower continued to increase, use of infantry lines diminished, until all infantry became light infantry in practice.

Modern classifications of infantry have expanded to reflect modern equipment and tactics, such as motorised infantry, mechanised or armoured infantry, mountain infantry, marine infantry, and airborne infantry.