So imagine it marching down a road. We can basically assume a typical country road with two lanes and two narrow shoulders between the ditches, because whatever the particulars, the narrowest stretch of road sets the pace. That's enough room for 4 men to march abreast, with one lane left free for other uses. The division will be spread out to allow extra space between each section, platoon, company, battalion and brigade, and, in total, will need about 1000 meters of road per thousand men. In total, then, a 12,000 man division will stretch out over a half-day's combat march. If the enemy is run into along the road, the division will come into battle in a single day.
That's why there are divisions in the first place. So that the army can be divided up by the number of available roads.
Okay, but imagine that the division is marching down, say, from the Ardennes plateau of Belgium towards the river Meuse. The high hills on the French side of the river dominate the scene, and the road follows a long and gentle slope. What's wrong with this picture? Well, in the Bastille Day parade of 1899, the French public was treated to a display of the latest military technology of the army of the Republic: the Soixante-Quinze. This new gun had a hydro-pneumatic recuperator that allowed it to fire without jumping out of position, a rapid-loading mechanism that allowed it to be fired as quickly as it recuperated, up to 20 rounds per minute with a fresh crew, and an automatic calculating sight, so that gun-aimers only needed to keep a telescopic sight laid on the target with a pair of adjusting dials. The new cordite propellant gave a muzzle velocity of 500 m/s, and the 16lb shrapnel shells, made thin-walled of the new high quality steels being made in recuperative, regenerative Siemens-style furnaces and machined by the new generation of powered machine tools, could hold 300 balls. (In the old days, a "shrapnel shell" contained a small amount of high explosive and a large number of musket-style balls that were "fired" out the front of the shell by a timed fuze.)
The result, provided that the initial aim was right, was that a single "75" on the French hills opposite, more than 5 kilometers away, could, without warning, open up on the advancing division with the equivalent firepower of 20 machine guns!
One often hears that the generals of 1914 underestimated the firepower of the machine gun. Bearing in mind that the 75 had been invented since the machine gun, these kinds of numbers tell us why. It didn't hold a candle to the new generation of artillery.
Obviously the solution to a problem like this was not to hope that it didn't happen, but rather to deploy the division in skirmish order at the rim of the valley so as not to give the enemy artillery this kind of target. Now, I've picked this specific example for its relevance to the Fall of France, but we should not get hung up on technology. From longbowmen on a hill facing the road exiting the forest of Crecy down to the a divisional artillery network controlled by a single radio-equipped forward observer in 1945, the tactical problem hasn't changed, only the scale. But scale is important here. If a bunch of longbowmen control 300 yards of road debouching from a forest, you either march your army up that road in a column and try to run the longbowmen over, or you stop and deploy into a long and shallow line that minimises your exposure while threatening the English flanks. If you figure that it will take an hour to shake out your lines in the middle of the woods, it will take an hour to get back into column. So the English can wait until you're deployed, and scamper. They've gained an hour in their retreat. If you need to disperse into skirmish formation and advance forward painfully slowly every time there might be a single enemy gun within 6000 (or, in 1940, 15,000) yards of your road, then all the enemy has to do is send a battery of guns forward, and it will have all the time in the world to get to Point B before you.
So there has to be another component to your solution. You have to make sure that the enemy isn't in that position. Military thinkers talk about "screens," and "security," and "reconnaissance," but it comes down to this. Somebody has to go see if the enemy is out there, and maybe shoot them if they are. And this means something crucial. You have to have a part of your army that is much faster than marching infantry, if the infantry are to march as fast as they can.
It's quite possible that we can name the man who brought this to the world's attention in 1595BC: Mursili I led a Hittite army all the way down to Babylon and sacked the city, ending the Amorite dynasty of Hammurabi and bringing back the temple idol of Marduk and the scribes who inadvertently started Indo-European by adapting the old Sumerian script to the mutable, pre-literate tongues of Anatolia. It's such an extraordinary and unprecedented achievement that it is hard to believe that it was not the war chariot's definitive baptism of fire. (A chariot "invention" event has been proposed for the Eurasian steppe in the late 2000s BC, but without rejecting the location of the event, Russian scholar Elena Kuzmina raises serious doubts about the chronology here.)
It's hard to emphasise more the world-historical importance of the horse, and thus our need to understand its agronomy. Attempts to sidestep the complex details by waves in the direction of compound bows and stirrups as all-explanatory devices not only do a disservice to world history, they help us understand the issues at stake in the Cavalry Division Debate, as I herewith christen it.
The first problem to appreciate is the matter of scale. How many horsemen would a divisional commander want to have under his command before he felt safe in making a pell-mell advance on Point B? Think about 3000 fresh-faced boys looking up in the last second of their lives at the whizzing shells about to bloom, and you realise that it is a trick question. The answer is, "all of them." That said, men on horses spread out more than men on foot. It's safe to say that a 30 year-old infantry officer can command 1--200 men, and a 35 year-old 600--1000, and the "company" and "battalion" organisations cluster around these strengths. Their cavalry equivalent are the squadron of 1--150 men, and the regiment (or "brigade" in the Nineteenth Century "Light Brigade" sense) is 4--600 sabres, as the old-timey military historians liked dto say to distinguish mounted men from foot soldiers ("rifles.") Above that, and you get into the realm of a cavalry division.
So, should a division commander settle for a squadron, which is to say, 150 sabres to screen the front and flanks of a column 15 kilometers long? No. Nor is he going to get a division, so he settles for a regiment. Or, hmm. Does he? The thing is, countries like Britain, France and Germany have huge populations of more than 40 millions and, as 1899 turns into 1900, mass conscription or some thought of imposing it. There's no reason for them to field fewer than 50 divisions, and the ambitious can aim at 100. 100 divisions x 500 horses/division=50,000 horses. Which does not seem so much until you remember that all the guns and wagons have to be drawn by horse as well. In fact, the Soixante-Quinze has set a new bar by requiring 6 horses. (Plus more additional horses to haul all that ammunition.)
And horses do not grow on trees. Great Britain, an unusually rich country with unusually rich farmers and a great deal of excellent stud land, has about 2 million horses. France, by contrast, a country with very little good stud land, has some trouble reaching a ratio of 1 horse/20 inhabitants. Nor can send your breeding stock to war, nor remove too many horses from agriculture. There is, in short, not likely to be enough horses to go round. (Except for Russia and India, which are plugged into the Inner Eurasian horse export market.)
So what are we going to do? For starters, infantry division commanders are not going to be allowed to have more than a squadron of horses or so. Many divisions will have their horses taken away entirely, especially if they aren't marching in the first wave. To provide additional security for the first wave, those horses will be concentrated in cavalry divisions and placed under army commanders. One can even imagine bolder, more sweeping notions. If Attila could ride across Europe and into France with an entire cavalry army, why could a German cavalry army not do the same, when one day in the future the inevitable war broke out between the two countries. (It did finally happen in the late summer of 1914 after the Schlieffen Plan had well and truly failed. And it worked out only a little better for the Germans than it did for Attila.)
The next thing you can try is find an alternative to the horses. Late Nineteenth Century technology was overflowing with possibilities. It is no coincidence that Count Zeppelin was a former cavalry officer, and across the border in France, Clement Adler got money for his experimental aeroplane from the French cavalry authorities. By the time 1914 rolled around, such divisional cavalry squadrons and regiments as were still about were stretched with companies of bicyclists, and armoured cars showed up as soon as war was declared, and promptly demonstrated the obvious problem that wheeled vehicles can't go where cavalry needs to go.
Nor were these visions the only ones. What if modern science could make foot soldiers as fast as horses? There's a great book by Rabinbach that covers these issues in pre-WWI France, and the Fascist-era's jogging soldiers, the celeri, are well known in their own right. In Britain, you have the cult of the Peninsular War-era Light Division, well developed by the middle of the Nineteenth Century and still going strong. It would be a little odd for the British to give up the advantage that their plentiful horseflesh conferred upon them, and so there was also the thought that rifleman+horse might turn out to be a superior form of warrior --a mounted rifleman. We don't really have a history of the idea of the mounted rifleman, although it has fascinated the English-speaking world since the 1840s. (Stephen Badsey is working to fix that.) Why? Well, the cavalry was seen as an aristocratic, Tory/Conservative, arm, while the riflemen were seen as a "middle class," Whig/Liberal arm. So it's all down to partisan politics --but if we take Daniel Szechi seriously, perhaps very consequential party politics.
So, next question. What happens when the horses go away, as they did in Britain during the interwar period, when the number fell to 200,000 by the mid-30s? One answer is that you say good riddance. You could make an absolutely brilliant field gun, far superior to the Soixante-Quinze, if you didn't have to worry about it being pulled by horses. A good tractor could pull a heavier load, faster, and never needed to be put out to grass to recover from a limp or whatnot. Okay, imagine that all of your wagon needs are being replaced by trucks. Wagons don't have to be hauling necessities about all the time. Usually, they end up loaded with footsore stragglers during most of the march to Point B. So systematise that: form the divisions' trucks up into a company, and have them lift --oh, say, an infantry brigade from the rear of the column to the front, then return down and pick up the new rear brigade and move it to the front...
And you get infantry that marches faster than cavalry. It still can't screen itself --but the point is, cavalry can't screen it, because the divisional cavalry falls behind. So that's it. The end of divisional (horse) cavalry. Also, the end of entire armies, as they march blindly into the fire ambush.
No, of course not. You need to put the cavalry in vehicles that are faster than horses. As I've already shown, there were a host of reasons for wanting to do that, anyway. It's just that by the mid-30s, it was inevitable. And if it was inevitable for the divisional cavalry, it was also inevitable for the British Army's Cavalry Division. To do the necessary screening work that the division had always done, it would need to be put into tanks.
Pretty straightforward? Right? Wrong. B. H. Liddell Hart threw a fit at the time, and had not recovered when he sat down to shape the history of World War II. Why? He had a host of arguments, of which the most telling in some people's minds is evidently the army's decision that the Inspector-General of Cavalry should be the commander of the division. Hart argued that the new "Mobile Division" should be under a tank man, who had ridden around on AFVs a lot, and thus understood them, whereas riding around on horses makes your brain freeze into arteriosclerotic stupidity.
The Army didn't make its decision in the dark. The IGC was to be the commander of the Cavalry Division for the same reason that commander of military districts would become GOCs of divisions upon mobilisation. He already had a headquarters and a staff, so he could organise the darn thing! If the particular IGC of 1937 or 1938 was too stupid to command an armoured division (and some proof, or even a name might be forthcoming from Hart), the army could always appoint another IGC. And since Hart's choice to command, Percy Hobart, was a arsehole and petty tyrant utterly unsuited to a field command, whatever his merits as an organiser, he is no-one to be throwing stones.
But in reality, Hart is just throwing up obstacles to obscure the real issues at stake.