Mechanised infantry
For example, in the period before the formation and in the initial period of the formation of the motorized rifle troops:

BTR-40 – in the period from 1950 to 1960s, 8,500 units were produced
BTR-50 — 1954 to 1970s – 6,500 pieces
BTR-152 — 1947 to 1962 – 12,421 pieces
BRDM-1 — 1957 to 1966 – 10,000 units
One or two motorised rifle regiments were also present in each tank division, and many tank regiments included one motorised rifle battalion.
After 1945, the Soviet Armed Forces and NATO further developed the equipment and doctrine for mechanized infantry. With the exception of airborne formations, the Red Army mechanized all its infantry formations. Initially, wheeled APCs, like the BTR-152, were used, some of which lacked overhead protection and were therefore vulnerable to artillery fire. It still gave the Soviet Army greater strategic flexibility because of the large land area and the long borders of the Soviet Union and its allies in the Warsaw Pact. Armored vehicles meant infantry were capable of overcoming water barriers and having means of protection against Weapons of Mass Destruction.
The US Army established the basic configuration of the tracked APC with the M75 and M59 before it adopted the lighter M113, which could be carried by Lockheed C-130 Hercules and other transport aircraft. The vehicle gave infantry the same mobility as tanks but with much less effective armor protection (it still had nuclear, biological, and chemical protection).
In the Vietnam War, the M113 was often fitted with extra armament and used as an ad hoc infantry fighting vehicle. Early operations by the Army of the Republic of Vietnam using the vehicle showed that troops were far more effective while they were mounted in the vehicles than when they dismounted. American doctrine subsequently emphasized mounted tactics. The Americans ultimately deployed a mechanized brigade and ten mechanized battalions to Vietnam.
The motorized rifle troops of the Soviet Armed Forces were the world's first infantry units that adopted a new class of combat vehicles in 1966 – Infantry fighting vehicles. BMP-1 began entering service in 1966. In the Federal Republic of Germany, an approximate analogue, the Marder, appeared only in 1970. Unlike the APC, which was intended merely to transport the infantry from place to place under armor, the IFV had heavy firepower that could support infantry. The Infantry fighting vehicle concept was subsequently copied by almost all countries of the world.
The introduction of the BMP-1 prompted the development of similar vehicles in Western armies, such as the West German Marder and American M2 Bradley. Many IFVs were also equipped with firing ports from which their infantry could fire their weapons from inside, but they were generally not successful and have been dropped from modern IFVs.
Soviet organization led to different tactics between the "light" and the "heavy" varieties of mechanized infantry. In the Soviet Army, a first-line "motor rifle" division from the 1970s onward usually had two regiments equipped with wheeled BTR-60 APCs and one with the tracked BMP-1 IFV. The "light" regiments were intended to make dismounted attacks on the division's flanks, while the BMP-equipped "heavy" regiment remained mounted and supported the division's tank regiment on the main axis of advance. Both types of infantry regiment still were officially titled "motor rifle" units.
A line of development in the Soviet Armed Forces from the 1980s was the provision of specialized IFVs for use by the Russian Airborne Troops. The first of them was the BMD-1, which had the same firepower as the BMP-1 but could be carried in or even parachuted from the standard Soviet transport aircraft. That made airborne formations into mechanized infantry at the cost of reducing their "bayonet" strength, as the BMD could carry only three or at most four paratroopers in addition to its three-man crew. They were used in that role in the Soviet invasion of Afghanistan in 1979.

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. In Operation Desert Shield, during the buildup phase of the First Gulf War, the U.S. Army was concerned about the lack of mobility, protection and firepower offered by existing rapid deployment (i.e., airborne) formations; and also about the slowness of deploying regular armored units. The experience led the U.S. Army to form combat brigades based on the Stryker wheeled IFV.
In the British Army, "heavy" units equipped with the Warrior IFV are described as "armoured infantry", and units with the Bulldog APC as "mechanised infantry". This convention is becoming widespread; for example the French Army has "motorisées" units equipped with the wheeled VAB and "mécanisées" units with the tracked AMX-10P.
The transport and other logistic requirements have led many armies to adopt wheeled APCs when their existing stocks of tracked APCs require replacement. An example is the Canadian Army, which has used the LAV III wheeled IFV in fighting in Afghanistan. The Italian, Spanish and Swedish armies are adopting (and exporting) new indigenous-produced tracked IFVs. The Swedish CV90 IFV in particular has been adopted by several armies.

A recent trend seen in the Israel Defense Forces and the Armed Forces of the Russian Federation is the development and introduction of exceptionally well-armored APCs (HAPC), such as the IDF Achzarit, that are converted from obsolete main battle tanks (such as the Soviet T-55). Such vehicles are usually 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 Chechen troops in Grozny during the First Chechen War in 1995.
Many APCs and IFVs currently under development are intended for rapid deployment by aircraft. New technologies that promise reduction in weight, such as electric drive, may be incorporated. However, facing a similar threat in post-invasion Iraq to that which prompted the Russians to convert tanks to APCs, the occupying armies have found it necessary to apply extra armor to existing APCs and IFVs, which adds to the overall size and weight. Some of the latest designs (such as the German Puma) are intended to allow a light, basic model vehicle, which is air-transportable, to be fitted in the field with additional protection, thereby ensuring both strategic flexibility and survivability.

Medium mechanized forces: In the late Cold War and early 21st century, various countries developed medium infantry forces armed with armored vehicles, which typically consisted of wheeled armored personnel carriers, infantry fighting vehicles, and assault guns. Medium mechanized forces are characterized by having more strategic air and road mobility than heavier, tank-based armored forces while offering better armor protection for the formation than the lighter motorized infantry formation, in which vehicles were considered "battle taxis" due to poor protection. The earliest experiment was the short-lived Soviet Light Motor Rifle Division in 1987, which consisted of wheeled BTR platforms for its primary armament. In the 1990s, the United States explored Stryker Brigade Combat Team (SBCT) formation and doctrines, which was a medium mechanized infantry formation with all-wheeled platforms centered around Stryker armored personnel carrier. In the early 21st century, China reformed its ground forces with the concept called Medium Combined Arms Brigade (CA-BDE), armed with Type 08 universal wheeled platform. A similar trend of adopting the medium mechanized forces was observed in European countries, including the Italian, Polish, and French armed forces.

Combined arms operations: It is generally accepted that single weapons system types are much less effective without the support of the full combined arms team; the pre-World War II notion of "tank fleets" has proven to be as unsound as the World War I idea of unsupported infantry attacks. Though many nations' armored formations included an organic mechanized infantry component at the start of World War II, the proportion of mechanized infantry in such combined arms formations was increased by most armies as the war progressed.
The lesson was re-learned, first by the Pakistani Army in the 1965 war with India, where the nation fielded two different types of armored divisions: one which was almost exclusively armor (the 1st), while another was more balanced (the 6th). The latter division showed itself to be far more combat-capable than the former.
Having achieved spectacular successes in the offensive with tank-heavy formations during the Six-Day War, the Israel Defense Forces found in the Yom Kippur War of 1973 that a doctrine that relied primarily on tanks and aircraft had proven inadequate. As a makeshift remedy, paratroopers were provided with motorized transport and used as mechanized infantry in coordination with the armor.

See also: Armoured warfare

Notes: 

Sources: Dunstan, Simon. Vietnam Tracks: Armor In Battle 1945–1975. 1982 edition, Osprey Publishing; ISBN 0-89141-171-2.
Starry, Donn A., General.