… the tank was a direct response to the battlefield conditions of WWI, in particular the trench stalemate on the Western front. The idea of some kind of armored “land cruiser” (potentially armed with machine guns) had been floated before WWI but never seriously considered and developed on, but serious development only began in 1915 with the formation of the Landship Committee early that year. Famously, they needed a code-name for their planned vehicle and opted first for “water carrier” and then for “tank”, thus giving the tank its peculiar English name.

And we should stop to note that as with any question of definition, this one too is language-sensitive. The exact confines of a term vary from one language to another; kampfpanzer, for instance is not necessarily an exact synonym for “tank”.

In any event, the basic demands of early tanks were dictated by the realities of the Western Front: a tank needed to be able to resist small arms fire (particularly machine guns), deliver direct supporting fire itself, it needed to be able to move on the muddy, artillery-flattened ground and it needed to be able to cross a trench. This last requirement – the need to be able to both climb a parapet (usually c. 4ft) and then cross over an 8ft wide trench – was significant in the design of early tanks.

Those factors in turn dictated a lot of the design of early tanks. The armor demands of resisting small arms fire meant that the vehicle would be heavy (and indeed, as soon as tanks appeared amongst Allied troops, their German opponents began introducing more powerful bullets, like the K bullet and later the 13.2mm anti-tank round fired from the Mauser 1918 T-Gewehr). And here is the first advantage of tracks. The weight of a vehicle is distributed along all of the area of contact it has with the ground; with tires that area is limited to the bottom of the tire so the total area of ground contact is fairly low, which is fine for most vehicles.

But tanks are heavy. Really heavy. Even something like the Renault FT could mass around 7 tons and by later standards that would be classified as a tankette (a “mini-tank” as it were); by WWII, medium tanks often clocked in around 30 tons. If you put a vehicle like that on tires, you are going to create a LOT of pressure on those small points of contact. That might still be OK if you are just going to drive on roads and other firm surfaces which can take the pressure. But remember: tanks were designed for the Western Front, which looks like this.

Fortunately for the landship committee, this wasn’t a new problem: farming tractors were also heavy and also had to operate in churned up (in this case, plowed) soft soil; the heaviest of these vehicles had much the same problem and the solution was continuous tracks or “treads”. When kept properly tensioned – tune in, by the by, to Nicholas “The Chieftain” Moran’s YouTube for more than you ever want to know about track tension – the track distributes the weight of the tank across the entire section of the track touching the ground, which reduces the ground pressure at any given point, allowing a big heavy tank to roll over terrain where even a much lighter wheeled vehicle would get stuck.

This is one of those points where the functionality of a tank (what a tank does) has such a strong influence on design that the design implications of the functionality become part of the definition: a tank has to be heavily armored and has to be able to move off-road and as a result has to be tracked, not wheeled. One might be able to imagine some sort of exotic technology that might make it possible to do all of the things a tank does without tracks, but we don’t have that yet.

The other factor was fire. I’ve mentioned this before, but one of the significant background factors of WWI is that a lot of the belligerents misjudged the kind of artillery they’d need for a general European war. Not to get too deep into the weeds here, but most of the belligerents expected a relatively rapid war of maneuver and so thought that light, direct-fire artillery like the famed French ’75 (the Matériel de 75mm Mle 1897) would be the most useful. Those guns could be moved quickly and could deliver a lot of quick firepower on static or moving formations of enemy infantry in support of friendly infantry.

The problem is that in the conditions of trench warfare, those guns – as they were configured, at least – were far less useful. They were, first off, much shorter in range which meant they had to be brought dangerously far forward to do their direct fire role – often so far forward they could be engaged by enemy rifles and machine guns. This was compounded by the fact that direct fire at range was ineffective against trench works (which are dug down into the earth). But at the same time, the value of rapid firing (because these lighter guns could fire a lot faster than the heavy, indirect fire artillery) direct fire artillery remained high, if only you could get it to the fight.

This was also a problem a tank could solve: as a mobile, armored platform it could move a rapid-firing direct fire gun forward without immediately being knocked out by enemy small arms to support the infantry. There is, I should note, early complexity on this point, with both “male” (heavy direct fire cannon focused) and “female” (machine gun focused) tanks in WWI though in the end “hermaphrodite” designs with both capabilities (but much more focus on the main cannon) triumph, so that’s what we’ll focus on.

And that gets us the fundamental role structure for tanks: enough armor to resist enemy small arms (but with the understanding that some weapons will always be effective against the tank), enough mobility to cross the churned up battlefield and some direct fire capability to support the infantry crossing it at the same time.

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