In the 1930's CE the French and especially German army created units of infantry that were equipped with half-tracks.
The French envisioned them to be used as counter-attackers, while the Germans used them to support rapidly advancing tanks.
In the middle of World War II they introduced "panzergrenadier" units.
Other countries quickly took up the idea and created mechanized infantry units of their own.
Motorized troops can travel quickly on roads but often get bogged down in heavy terrain.
The half-tracks of the mechanized troops are able to deal with that and keep up with the tanks.
The first half-tracks were later replaced by fully tracked and all-terrain vehicles.
All of them are lightly armored to protect the troops inside from small arms fire, though they cannot withstand hits by heavy guns.
Therefore the infantry vehicles are called armored personnel carriers.
During the Cold War, some infantry vehicles were upgraded with heavier armor, guns and even anti-tank weapons, becoming something like light tanks.
These vehicles can lend fire support to their crews operating on foot, or even fight without unloading them, thus blurring the distinction between infantry and armor.
In the 1950's CE the German and French armies created the first such infantry fighting vehicles.
The Soviet BMP-1, introduced in 1966 CE, is generally considered to be the first mature member of this class.
Again other countries soon followed suit.
Mechanized infantry is more flexible and effective than motorized infantry, but also more expensive.
It occupies a middle ground between foot infantry and tanks.
They are one of the most flexible types of units on the battlefield today, though only effective when used well in combined arms tactics.
War Matrix - Mechanized infantry
World Wars 1914 CE - 1945 CE, Weapons and technology