The period between WWI and WWII – the “interwar” period – was a period of broad experimentation with tank design and so by the time we get to WWII there are a number of sub-groupings of tanks. Tanks could be defined by weight or by function. The main issue in both cases was the essential tradeoff between speed, firepower and armor: the heavier you made the armor and the gun the heavier and thus slower the tank was. The British thus divided their tank designs between “cruiser tanks” which were faster but lighter and intended to replace cavalry while the “infantry tanks” were intended to do the role that WWI tanks largely had in supporting infantry advances. Other armies divided their tanks between “light”, “medium”, and “heavy” tanks (along with the often designed but rarely deployed “super heavy” tanks).

What drove the differences in tank development between countries were differences between how each of those countries imagined using their tanks, that is differences in tank doctrine. Now we should be clear here that there were some fundamental commonalities between the major schools of tank thinking: in just about all cases tanks were supposed to support infantry in the offensive by providing armor and direct fire support, including knocking out enemy tanks. Where doctrine differed is exactly how that would be accomplished: France’s doctrine of “Methodical Battle” generally envisaged tanks moving at the speed of mostly foot infantry and being distributed fairly evenly throughout primarily infantry formations. That led to tanks that were fairly slow with limited range but heavily armored, often with just a one-man turret (which was a terrible idea, but the doctrine reasoned you wouldn’t need more in a slow-moving combat environment). Of course this worked poorly in the event.

More successful maneuver warfare doctrines recognized that the tank needed infantry to perform its intended function (it has to have infantry to support) but that tanks could now move fast enough and coordinate well enough (with radios) that any supporting arms like infantry or artillery needed to move a lot faster than walking speed to keep up. Both German “maneuver warfare” (Bewegungskrieg) and Soviet “Deep Operations” (or “Deep Battle”) doctrine saw the value in concentrating their tanks into powerful striking formations that could punch hard and move fast. But tanks alone are very vulnerable and in any event to attack effectively they need things like artillery support or anti-air protection. So it was necessary to find ways to allow those arms to keep up with the tanks (and indeed, a “Panzer divsion” is not only or even mostly made up of tanks!).

At the most basic level, one could simply put the infantry on trucks or other converted unarmored civilian vehicles, making “motorized” infantry, but […] part of the design of tanks is to allow them to go places that conventional civilian vehicles designed for roads cannot and in any event an unarmored truck is a large, vulnerable tempting target on the battlefield.

The result is the steady emergence of what are sometimes jokingly called “battle taxis” – specialized armored vehicles designed to allow the infantry to keep up with the tanks so that they can continue to be mutually supporting, while being more off-road capable and less vulnerable than a truck. In WWII, these sorts of vehicles were often “half-tracks” – semi-armored, open-topped vehicles with tires on the front wheels and tracks for the back wheels, though the British “Universal Carrier” was fully tracked. Crucially, while these half-tracks might mount a heavy machine gun for defense, providing fire support was not their job; being open-topped made them particularly vulnerable to air-bursting shells and while they were less vulnerable to fire than a truck, they weren’t invulnerable by any means. The intended use was to deposit infantry at the edge of the combat area, which they’d then move through on foot, not to drive straight through the fight.

The particular vulnerability of the open-top design led to the emergence of fully-enclosed armored personnel carriers almost immediately after WWII in the form of vehicles like the M75 Armored Infantry Vehicle (though the later M113 APC was eventually to be far more common) and the Soviet BTRs (“Bronetransporter” or “armored transport”), beginning with the BTR-40; Soviet BTRs tended to be wheeled whereas American APCs tend to be tracked, something that also goes for their IFVs (discussed below). These vehicles often look to a journalist or the lay observer like a tank, but they do not function like tanks. The M113 APC, for instance, has just about 1.7 inches of aluminum-alloy armor, compared to the almost four inches of much heavier steel armor on the contemporary M60 “Patton” tank. So while these vehicles are armored, they are not intended to stick in the fight and are vulnerable to much lighter munitions than contemporary tank would be.

At the same time, it wasn’t just the infantry that needed to be able to keep up: these powerful striking units (German Panzer divisions, Soviet mechanized corps or US armored divisions, etc.) needed to be able to also bring their heavy weaponry with them. At the start of WWII, artillery, anti-tank guns and anti-air artillery remained almost entirely “towed” artillery – that is, it was pulled into position by a truck (or frequently in this period still by horses) and emplaced (“unlimbered”) to be fired. Such systems couldn’t really keep up with the tanks they needed to support and so we see those weapons also get mechanized into self-propelled artillery and anti-air (and for some armies, tank destroyers, although the tank eventually usurps this role entirely).

Self-propelled platforms proved to have another advantage that became a lot more important over time: they could fire and then immediately reposition. Whereas a conventional howitzer has to be towed into position, unlimbered, set up, loaded, fired, then limbered again before it can move, something like the M7 Priest can drive itself into position, fire almost immediately and then immediately move. This maneuver, called “shoot-and-scoot” (or, more boringly, “fire-and-displace”) enables artillery to avoid counter-battery fire (when an army tries to shut down enemy artillery by returning fire with its own artillery). As artillery got more accurate and especially with the advent of anti-artillery radars, being able to shoot-and-scoot became essential.

Now while self-propelled platforms were tracked (indeed, often using the same chassis as the tanks they supported), they’re not tanks. They’re designed primarily for indirect fire (there is, of course, a sidebar to be written here on German “assault guns” – Sturmgeschütz – and their awkward place in this typology, but let’s keep it simple), that is firing at a high arc from long range where the shell practically falls on the target and thus are expected to be operating well behind the lines. Consequently, their armor is generally much thinner because they’re not designed to be tanks, but to play the same role that towed artillery (or anti-air, or rocket artillery, etc.) would have, only with more mobility.

So by the end of WWII, we have both tanks of various weight-classes, along with a number of tank-like objects (APCs, self-propelled artillery and anti-air) which are not tanks but are instead meant to allow their various arms to keep up with the tanks as part of a combined arms package.

Bret Devereaux, “Collections: When is a ‘Tank’ Not a Tank?”, A Collection of Unmitigated Pedantry, 2022-05-06.