Mechanized infantry
History: As early as 1915 the British instigated a tracked vehicle that could carry 50 equipped troops under armour but the project got no further than trials before cancellation. Some of the first mechanized infantry were German assault teams mounted on A7V tanks during World War I. The vehicles were extra-large to let them carry sizeable assault teams and would regularly carry infantry on board in addition to their already large crews that were trained as stormtroopers. All machine-gun-armed A7V tanks carried two small flamethrowers for their dismounts to use. A7V tank would often carry a second officer to lead the assault team.
During the Battle of St. Quentin in late March 1918, A7Vs were accompanied by twenty stormtroopers from Rohr Assault Battalion, but it is unspecified if they were acting as dismounts or were accompanying the tanks on foot. During the battle, tank crews were reported to have dismounted and attacked enemy positions with grenades and flamethrowers on numerous occasions.
Another example of the use of such a method of fighting is the capture of Villers-Bretonneux, in which A7Vs would suppress the defenders with machine gun fire and assault teams would dismount and attack them with grenades.
The British heavy tank design was given an extended hull to cross wide German trenches. This Mark V** had space for fourteen troops. The Mark IX tank based on the Mark V was designed solely for carrying troops with space for 30 but the war ended before the order was complete and they could be used.
Towards the end of World War I, all the armies involved were faced with the problem of maintaining the momentum of an attack. Tanks, artillery, or infiltration tactics could all be used to break through an enemy defense, but almost all offensives launched in 1918 ground to a halt after a few days. The following infantry quickly became exhausted, and artillery, supplies and fresh formations could not be brought forward over the battlefields quickly enough to maintain the pressure on the regrouping enemy forces.
It was widely acknowledged that cavalry was too vulnerable to be used on most European battlefields, but many armies continued to deploy them. Motorized infantry could maintain rapid movement, but their trucks required either a good road network or firm open terrain, such as desert. They were unable to traverse a battlefield obstructed by craters, barbed wire, and trenches. Tracked or all-wheel drive vehicles were to be the solution.
Following the war, development of mechanized forces was largely theoretical for some time, but many nations began rearming in the 1930s. The British Army had established an Experimental Mechanized Force in 1927, but it failed to pursue that line because of budget constraints and the prior need to garrison the frontiers of the British Empire.
Although some proponents of mobile warfare, such as J. F. C. Fuller, advocated building "tank fleets", other, such as Heinz Guderian in Germany, Adna R. Chaffee Jr. in the United States, and Mikhail Tukhachevsky in the Soviet Union, recognized that tank units required close support from infantry and other arms and that such supporting arms needed to maintain the same pace as the tanks.
As the Germans rearmed in the 1930s, they equipped some infantry units in their new Panzer divisions with the half-track Sd.Kfz. 251, which could keep up with tanks on most terrain. The French Army also created "light mechanized" (légère mécanisée) divisions in which some of the infantry units possessed small tracked carriers. Together with the motorization of the other infantry and support units, this gave both armies highly mobile combined-arms formations. The German doctrine was to use them to exploit breakthroughs in Blitzkrieg offensives, whereas the French envisaged them being used to shift reserves rapidly in a defensive battle.

World War II: As World War II progressed, most major armies integrated tanks or assault guns with mechanized infantry, as well as other supporting arms, such as artillery and combat engineers, as combined arms units.
Allied armored formations included a mechanized infantry element for combined arms teamwork. For example, US armored divisions had a balance of three battalions each of tanks, armored infantry, and self-propelled artillery. The US armored infantry was fully equipped with M2 and M3 halftracks. In the British and Commonwealth armies, "Type A armoured brigades," intended for independent operations or to form part of armored divisions, had a "motor infantry" battalion mounted in Universal Carriers or later in lend-lease halftracks. "Type B" brigades lacked a motor infantry component and were subordinated to infantry formations.
The Canadian Army and, subsequently the British Army, used expedients such as the Kangaroo APC, usually for specific operations rather than to create permanent mechanized infantry formations. The first such operation was Operation Totalize in the Battle of Normandy, which failed to achieve its ultimate objectives but showed that mechanized infantry could incur far fewer casualties than dismounted troops in set-piece operations.
The German Army, having introduced mechanized infantry in its Panzer divisions, later named them Panzergrenadier units. In the middle of the war, it created entire mechanized infantry divisions and named Panzergrenadier divisions.
Because the German economy could not produce adequate numbers of its half-track APC, barely a quarter or a third of the infantry in Panzer or Panzergrenadier divisions were mechanized, except in a few favored formations. The rest were moved by truck. However, most German reconnaissance units in such formations were also primarily mechanized infantry and could undertake infantry missions when it was needed. The Allies generally used jeeps, armored cars, or light tanks for reconnaissance.
The Red Army began the war while still in the process of reorganizing its armored and mechanized formations, most of which were destroyed during the first months of the German Invasion of the Soviet Union. About a year later, the Soviets recreated division-sized mechanized infantry units, termed mechanized corps, usually with one tank brigade and three mechanized infantry brigades, with motorized supporting arms. They were generally used in the exploitation phase of offensives, as part of the prewar Soviet concept of deep operations.
The Soviet Army also created several cavalry mechanized groups in which tanks, mechanized infantry and horsed cavalry were mixed. They were also used in the exploitation and pursuit phases of offensives. Red Army mechanized infantry were generally carried on tanks or trucks, with only a few dedicated lend-lease half-track APCs.
The New Zealand Army ultimately fielded a division of a roughly similar composition to a Soviet mechanized corps, which fought in the Italian Campaign, but it had little scope for mobile operations until near the end of the war.
The Romanian Army fielded a mixed assortment of vehicles. These amounted to 126 French-designed Renault UE Chenillettes which were licence-built locally, 34 captured and refurbished Soviet armored tractors, 27 German-made armored half-tracks of the Sd.Kfz. 250 and Sd.Kfz. 251 types, over 200 Czechoslovak Tatra, Praga and Skoda trucks (the Tatra trucks were a model which was specifically built for the Romanian Army) as well as 300 German Horch 901 4x4 field cars. Sd.Kfz. 8 and Sd.Kfz. 9 half-tracks were also acquired, as well as nine vehicles of the Sd.Kfz. 10 type and 100 RSO/01 fully tracked tractors. The Romanians also produced five prototypes of an indigenous artillery tractor.

Cold War: On July 9, 1945, Decree of the State Defence Committee No. GKO-9488ss,  "On the Resupply of Armored and Mechanized Forces of the Red Army" was issued. It ordered the creation of mechanised divisions from many rifle divisions, included in the Armored and Mechanised Troops. In some cases, cavalry divisions and airborne divisions also became mechanised divisions The Soviet motorised rifle troops officially appeared in accordance with the Directive of the Minister of Defense of the USSR No. org. / 3/62540 of February 27, 1957. This directive ordered part of the mechanized divisions and all rifle units and formations reorganized into 'motorised rifle' in the period 1957 to 1964. Creation of the motorised rifle troops was facilitated by large-scale mechanisation of the whole Soviet Ground Forces.
This became possible due to the increase in the production of armored personnel carriers, self-propelled guns and so on. 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.