A man can walk four miles in an hour relatively easily. And yet, an army of foot soldiers marching (in the ancient world) would cover perhaps six miles in a whole day. Even horsemen would only go twelve.
There are exceptions, but the above are averages taken from Theodore Dodge’s excellent histories (check out his Hannibal, Alexander and Caesar biographies if you haven’t yet).
Why does it take so long to move an army, when a single man with staging posts (for fresh horses) could cover, theoretically at least, well over a hundred miles in a single day? Even a chap out for a walk could easily make 10-20 miles over the course of a day.
Various changeable circumstances can affect how fast an army can move. Weather, terrain, and supplies all have a serious impact (more on those below). But even when it’s nice and warm, the ground’s flat and roads are good, and there’s plenty of food and beer, armies are still, usually, horrendously slow.
Moving one person is easy. They get up on time, and wander off. If they reach a bridge, that’s fine. If they need to climb a little, that’s usually no problem.
An army is different. The whole army can’t set off at once, because a road might only be wide enough for six, or fewer, to march abreast. Even as the vanguard strolls off, most of the rest of the army will be taking down last night’s camp and eating the last of the cheese. The sheer volume of people slows the army’s progress.
The number of men and beasts (not just war horses, but donkeys and oxen and mules to carry supplies or pull wagons) can also ruin roads. What might be a nice journey for the vanguard could be a squelching quagmire for the middle or rearguard. Similarly, if the vanguard reaches difficulties (say a flood washes away the only bridge for miles and it needs repairing) that then slows everyone else as a queue forms.
Narrow passes in mountains or slender footbridges are no problem for one man, but they’re bottlenecks when you’ve got thousands. Not only that, they may well be impassable for wagons and difficult/impossible for horses. A route one man can take is not necessarily a route an army can take.
Then there’s pestilence. Leave aside that a small army of whores will be prising coin from men who could die tomorrow (pox was spread thus although certain diseases were different. Syphilis didn’t exist in medieval England, arriving in the Tudor period and only mutating in Elizabeth I’s reign into the disease it is today). Medieval hygiene could include eating in close proximity to latrines. The camp disease of dysentery would usually break out. Fouled wells or even just drinking uncontaminated water could lead to typhoid. Having so many men together (and a medieval army could outnumber most medieval cities’ populations) in such close proximity massively increased the chances of disease breaking out, and then spreading rapidly. For this reason, armies besieging a castle/city could suffer as much as those trapped on the inside.
Supplies were often problematic. Gathering sufficient before you start depended on a good harvest and organisational abilities. If your adversary knows you’re coming he’ll foul wells and ensure harvested crops are safe inside castle walls so you struggle to feed your army off the land. This means the foraging parties have to roam further afield (and they need protection so you need to send more men), again slowing an army down. One man can swipe a few apples and blackberries, but an army takes a lot of feeding (and the animals need food too).
Weather can have a substantial impact. Ordinary drizzle (almost the default setting of Britain) can soften roads which turn to sludge beneath a thousand marching feet. Heavier rain can destroy roads or bridges, or flood camps and drown people and horses. But hot weather has dangers too. Finding sufficient water becomes even harder, and may slow the pace of men and animals. Even worse, forest fires (as now) can spring up out of nowhere.
Most travelling in the ancient and medieval world, as you’d expect, was by land. However, sea journeys also could be delayed on account of an army. If you don’t have enough ships because they’re delayed due to bad weather or simply take time to arrive, then either you split an army in two and risk it being defeated in detail, or you have to wait. One man needs just one ship.
In books, both historical and fictional, it’s entirely legitimate to have individuals travel a lot faster than armies, for all sorts of reasons (not to mention the possibility of messages being sent by bird).
As an aside, the Persians had an interesting measure called the parasang. Unlike a mile, the parasang was a unit of distance measured not in length but time. One parasang was one day’s march. That’s quite a clever way of doing things, as two roads leading to the same place might have a very large number of miles to the north compared to the south, but if the south road leads through mountains the northern road might still be a quicker route.
PS the next few blogs will likely be book reviews of The Wonder Book of Aircraft, The Emperor’s Edge, and Spies, Sadists and Sorcerers.