Infantry
When combat is expected, infantry typically switch to "packing light", meaning reducing their equipment to weapons, ammunition, and other basic essentials, and leaving other items deemed unnecessary with their transport or baggage train, at camp or rally point, in temporary hidden caches, or even (in emergencies) simply discarding the items. Additional specialised equipment may be required, depending on the mission or to the particular terrain or environment, including satchel charges, demolition tools, mines, or barbed wire, carried by the infantry or attached specialists.
Historically, infantry have suffered high casualty rates from disease, exposure, exhaustion and privation — often in excess of the casualties suffered from enemy attacks. Better infantry equipment to support their health, energy, and protect from environmental factors greatly reduces these rates of loss, and increase their level of effective action. Health, energy, and morale are greatly influenced by how the soldier is fed, so militaries issue standardised field rations that provide palatable meals and enough calories to keep a soldier well-fed and combat-ready.
Communications gear has become a necessity, as it allows effective command of infantry units over greater distances, and communication with artillery and other support units. Modern infantry can have GPS, encrypted individual communications equipment, surveillance and night vision equipment, advanced intelligence and other high-tech mission-unique aids.
Armies have sought to improve and standardise infantry gear to reduce fatigue for extended carrying, increase freedom of movement, accessibility, and compatibility with other carried gear, such as the American All-purpose Lightweight Individual Carrying Equipment (ALICE).

Weapons: Infantrymen are defined by their primary arms – the personal weapons and body armour for their own individual use. The available technology, resources, history, and society can produce quite different weapons for each military and era, but common infantry weapons can be distinguished in a few basic categories.

Ranged combat weapons: javelins, slings, blowguns, bows, crossbows, hand cannons, arquebuses, muskets, grenades, flamethrowers.
Melee combat weapons: bludgeoning weapons like clubs, flails and maces; bladed weapons like swords, daggers, and axes; pole weapons like spears, halberds, naginata, and pikes.
Both ranged and close weapons: the bayonet fixed to a firearm allows infantrymen to use the same weapon for both ranged combat and close combat. This started with muskets and its use still continues with modern assault rifles. Use of the bayonet has declined with the introduction of automatic firearms, but are still generally kept as a weapon of last resort.
Infantrymen often carry secondary or back-up weapons, sometimes called a sidearm or ancillary weapons. Infantry with ranged or pole weapons often carried a sword or dagger for possible hand-to-hand combat. The pilum was a javelin the Roman legionaries threw just before drawing their primary weapon, the gladius (short sword), and closing with the enemy line.
Modern infantrymen now treat the bayonet as a backup weapon, but may also have handguns as sidearms. They may also deploy anti-personnel mines, booby traps, incendiary, or explosive devices defensively before combat.

Protection: Infantry have employed many different methods of protection from enemy attacks, including various kinds of armour and other gear, and tactical procedures.
The most basic is personal armour. This includes shields, helmets and many types of armour – padded linen, leather, lamellar, mail, plate, and kevlar. Initially, armour was used to defend both from ranged and close combat; even a fairly light shield could help defend against most slings and javelins, though high-strength bows and crossbows might penetrate common armour at very close range. Infantry armour had to compromise between protection and coverage, as a full suit of attack-proof armour would be too heavy to wear in combat.

As firearms improved, armour for ranged defence had to be made thicker and heavier, which hindered mobility. With the introduction of the heavy arquebus designed to pierce standard steel armour, it was proven easier to make heavier firearms than heavier armour; armour transitioned to be only for close combat purposes. Pikemen armour tended to be just steel helmets and breastplates, and gunners had very little or no armour at all. By the time of the musket, the dominance of firepower shifted militaries away from any close combat, and use of armour decreased, until infantry typically went without wearing any armour.
Helmets were added back during World War I as artillery began to dominate the battlefield, to protect against their fragmentation and other blast effects beyond a direct hit. Modern developments in bullet-proof composite materials like kevlar have started a return to body armour for infantry, though the extra weight is a notable burden.
In modern times, infantrymen must also often carry protective measures against chemical and biological attack, including military gas masks, counter-agents, and protective suits. All of these protective measures add to the weight an infantryman must carry, and may decrease combat efficiency.

Infantry-served weapons: Early crew-served weapons were siege weapons, like the ballista, trebuchet, and battering ram. Modern versions include machine guns, anti-tank missiles, and infantry mortars.

Formations: Beginning with the development the first regular military forces, close-combat regular infantry fought less as unorganised groups of individuals and more in coordinated units, maintaining a defined tactical formation during combat, for increased battlefield effectiveness; such infantry formations and the arms they used developed together, starting with the spear and the shield.
A spear has decent attack abilities with the additional advantage keeping opponents at distance; this advantage can be increased by using longer spears, but this could allow the opponent to side-step the point of the spear and close for hand-to-hand combat where the longer spear is near useless. This can be avoided when each spearman stays side by side with the others in close formation, each covering the ones next to him, presenting a solid wall of spears to the enemy that they cannot get around.
Similarly, a shield has decent defence abilities, but is literally hit-or-miss; an attack from an unexpected angle can bypass it completely. Larger shields can cover more, but are also heavier and less manoeuvrable, making unexpected attacks even more of a problem. This can be avoided by having shield-armed soldiers stand close together, side-by-side, each protecting both themselves and their immediate comrades, presenting a solid shield wall to the enemy.

The opponents for these first formations, the close-combat infantry of more tribal societies, or any military without regular infantry (so called "barbarians") used arms that focused on the individual – weapons using personal strength and force, such as larger swinging swords, axes, and clubs. These take more room and individual freedom to swing and wield, necessitating a more loose organisation. While this may allow for a fierce running attack (an initial shock advantage) the tighter formation of the heavy spear and shield infantry gave them a local manpower advantage where several might be able to fight each opponent.
Thus tight formations heightened advantages of heavy arms, and gave greater local numbers in melee. To also increase their staying power, multiple rows of heavy infantrymen were added. This also increased their shock combat effect; individual opponents saw themselves literally lined-up against several heavy infantryman each, with seemingly no chance of defeating all of them. Heavy infantry developed into huge solid block formations, up to a hundred meters wide and a dozen rows deep.
Maintaining the advantages of heavy infantry meant maintaining formation; this became even more important when two forces with heavy infantry met in battle; the solidity of the formation became the deciding factor. Intense discipline and training became paramount. Empires formed around their military.

Organization: The organization of military forces into regular military units is first noted in Egyptian records of the Battle of Kadesh (c. 1274 BC). Soldiers were grouped into units of 50, which were in turn grouped into larger units of 250, then 1,000, and finally into units of up to 5,000 – the largest independent command. Several of these Egyptian "divisions" made up an army, but operated independently, both on the march and tactically, demonstrating sufficient military command and control organisation for basic battlefield manoeuvres. Similar hierarchical organizations have been noted in other ancient armies, typically with approximately 10 to 100 to 1,000 ratios (even where base 10 was not common), similar to modern sections (squads), companies, and regiments.

Training: The training of the infantry has differed drastically over time and from place to place. The cost of maintaining an army in fighting order and the seasonal nature of warfare precluded large permanent armies.
The antiquity saw everything from the well-trained and motivated citizen armies of Greece and Rome, the tribal host assembled from farmers and hunters with only passing acquaintance with warfare and masses of lightly armed and ill-trained militia put up as a last ditch effort. Kushite king Taharqa enjoyed military success in the Near East as a result of his efforts to strengthen the army through daily training in long-distance running.
In medieval times the foot soldiers varied from peasant levies to semi-permanent companies of mercenaries, foremost among them the Swiss, English, Aragonese and German, to men-at-arms who went into battle as well-armoured as knights, the latter of which at times also fought on foot.
The creation of standing armies—permanently assembled for war or defence—saw increase in training and experience. The increased use of firearms and the need for drill to handle them efficiently.