Mechanized infantry are infantry troops provided with trucks, armored personnel carriers (APCs), or infantry fighting vehicles (IFVs) for transport and combat. Their support weapons are also provided with motorized transport, or else built directly into combat vehicles, in order to keep pace with the mechanized infantrymen in combat. For units equipped with most types of APC or any type of IFV, fire support weapons such as machineguns, autocannons, or small-bore direct-fire howitzers are often mounted directly on the infantry's own transport vehicles.
History
Mechanized infantry was introduced between World War I and World War II, when French and later German infantry were first provided with trucks or custom built half-tracked and motorized transport for their support units, resulting in highly mobile formations that could keep pace with armored formations when exploiting breakthroughs in Blitzkrieg-style offensives.
World War II
As World War II progressed, the Germans integrated tanks or assault guns with mechanized infantry as combined arms Panzergrenadier Divisions, providing mobile anti-tank defense and close-up direct fire support for the infantry.
Most other armies of the period also fielded mechanized infantry in units up to brigade in size. Their armored divisions and some armored brigades also included a mechanized infantry element for combined arms support. For example, British armoured brigades had a motor infantry battalion mounted in Bren carriers. Later in the war, the US Army used large numbers of M3 Half-track vehicles to give their infantry mobility. The British and Canadians also used expedients such as the Kangaroo APC.
Like the Germans, the Soviet army fielded division-sized mechanized infantry units which they called Mechanized Corps, usually consisting of one tank brigade and three mechanized infantry brigades, plus artillery and other support units. New Zealand fielded a division that was sent to Italy, with an organization intermediate between an armored division and a mechanized infantry division.
The Cold War
In the post-war era, the Soviet Red Army and NATO further developed the equipment and doctrine for mechanized infantry. Though not the first tracked APC, the American M113 was the first in widespread use which gave the infantry the same mobility and almost the same protection as tanks (including against Nuclear, Biological and Chemical, or NBC threats). Even more important was the Russian BMP-1, which was the first Infantry Fighting Vehicle. Unlike the APC which merely transported the infantry from place to place, the IFV possessed firepower which could support the infantry in attack or defence. The introduction of this vehicle prompted the development of similar vehicles in Western armies, such as the German Marder and US M2 Bradley.
This led to different tactics between the "light" and "heavy" varieties of mechanized infantry. In the Soviet Army, a first-line "Motor Rifle" division usually had two regiments equipped with the wheeled BTR-60 APC and one with the tracked BMP-1 IFV. The "light" regiments were intended to make dismounted attacks on the division's flanks, supported by artillery; the BMP-equipped "heavy" regiment supported the division's tank regiment on the main axis of advance. Both types of infantry regiment nevertheless were officially titled "Motor Rifle" units.
A line of development in the Soviet Armed Forces was the provision of specialised IFV's for use by their airborne forces. The first of these was the BMD-1, which had the same firepower as the BMP-1, but which could be carried in or even parachuted from the standard Russian transport aircraft. This made airborne formations almost into mechanized infantry. They were used in this role in the invasion of Afghanistan in 1979. This trend was not followed by Western nations.
The Present Day
At present, almost all infantry units from industrialized nations are provided with some type of motor transport. Infantry units equipped with IFVs rather than lighter vehicles are commonly designated as "heavy", indicating more combat power but also more costly long-range transportation requirements.
The transport and other logistic requirements have led many armies to adopt wheeled APC's when their existing stocks of tracked APC's require replacement. An example is the Canadian Army. The US Army is also following this trend, forming brigades which use the Stryker wheeled IFV (although they will continue to field "heavy" formations for many years to come).
Armoured Infantry
In the British Army, units equipped with the Warrior IFV are described as "Armoured Infantry", units with the wheeled Saxon APC as "Mechanised Infantry". This convention is becoming widespread; for example the French Army has "Mécanisée" units equipped with the wheeled VAB and "Blindée" (armoured) units with the tracked AMX-10P.
Conversion of MBTs
A recent trend, seen in the armies of Israel and the Russian Federation is the introduction of exceptionally well-armoured APC's such as the IDF Achzarit which are converted from obsolete Main Battle Tanks (such as the Russian T-55). Such vehicles are expedients, and lack of space prevents the armament of an IFV being carried in addition to an infantry section or squad. In the Russian Army, such vehicles were introduced for fighting in urban areas, where the risk from short range infantry anti-tank weapons such as the RPG-7 is highest, after Russian tank and motor infantry units suffered heavy losses fighting insurgents in Grozny in 1995.
As support for armored formations
It has been discovered repeatedly that armored formations are much less effective without the support of infantrymen; the pre-WWII notion of "tank fleets" has not proved to be sound. Though many nations' armored formations included an organic mechanized infantry component at the start of the war, the proportion of mechanized infantry in such combined arms formations was increased by most armies as the war progressed.
The lesson was re-learned by the Israeli Defense Force in the 1973 war, when a doctrine that relied primarily on tanks, paratroopers, and aircraft proved inadequate. As a makeshift remedy paratroopers were provided with motorized transport and used as mechanized infantry in coordination with the armor; after the war the IDF reorganized its armored formations on the basis of the lesson learned.
The doctrine is now universal in nations capable of supporting armored formations. Recently the US Army has fielded both armored divisions and "heavy" infantry divisions; the organization of the two types was almost identical, except that the ratio of armored battalions to mechanized infantry battalions is slightly higher in the armored divisions and slightly lower in the heavy infantry divisions.