Infantry
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.
The introduction of national and mass armies saw an establishment of minimum requirements and the introduction of special troops (first of them the engineers going back to medieval times, but also different kinds of infantry adopted to specific terrain, bicycle, motorcycle, motorised and mechanised troops) culminating with the introduction of highly trained special forces during the first and second World War.

Air force and naval infantry: Naval infantry, commonly known as marines, are primarily a category of infantry that form part of the naval forces of states and perform roles on land and at sea, including amphibious operations, as well as other, naval roles. They also perform other tasks, including land warfare, separate from naval operations.
Air force infantry and base defense forces, such as the United States Air Force Security Forces, Royal Air Force Regiment, Royal Australian Air Force Airfield Defence Guards, and Indonesian Air Force Paskhas Corps are used primarily for ground-based defense of air bases and other air force facilities. They also have a number of other, specialist roles. These include, among others, Chemical, Biological, Radiological and Nuclear (CBRN) defence and training other airmen in basic ground defense tactics.

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Citations: Infentory

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External links: 
Historic films and photos showing Infantries in World War I at europeanfilmgateway.eu
In Praise of Infantry, by Field-Marshal Earl Wavell; First published in "The Times", Thursday, 19 April 1945.
The Lagunari "Serenissima" Regiment KFOR: KFOR Chronicle.
Web Version of U.S. Army Field Manual 3–21.8 – The Infantry Rifle Platoon and Squad.
"Infantry" . Encyclopædia Britannica. Vol. 14 (11th ed.). 1911. pp. 517–533. — includes several drawings