# Armies Crossing Mountains



## Centerfield97 (Feb 5, 2012)

So, in my WIP i have my primary city threatened by a large, impending army travelling through a substantial mountain range to conquer it in its weakened state.

How long would a large army (20,000-70,000) complete with mammoths, provisions, etc. take to cross a mountain range?  The only historical example I can think of is Hannibal crossing the Alps.  How long was that journey, and what were the casualties like?


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## Drakhov (Feb 5, 2012)

A lot would depend on the mountains themselves, the availability of passes through them, the weather and if they have to fight their way through.  Wiki doesn't state how long it took Hannibal to cross the Alps specifically - apparently he set out from New Carthage (SE Spain) in Spring 218BC (fighting his way through Spain and Gaul) and had 40000 infantry and 12000 cavalry when he entered Gaul, by the time he reached the Pyrenees numbered 38000 infantry, 8000 cavalry and 37 elephants, and entering Italy with 20000 infantry, 4000 cavalry and a handful of elephants - 50% casualties if these figures are to believed - but that includes the whole journey and battles fought along the way, not just crossing the Alps - Wiki also says he left 11000 Iberian (Spanish) troops as a garrison before crossing into Gaul.

He fought his first battle after the Alps crossing (The Battle of Trebia) in December 218 BC so 6-8 months after he left New Carthage.


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## Kelise (Feb 5, 2012)

I keep reading that as 'Animal Crossing'.

Like Drakhov says, we'd need quite a bit more information as to how long it would take. It could take a month, or it could take six, or more. How well trained is the army? Is there a worn path? Has the path changed thanks to land/snow slides? Are they being led by someone who's done the journey many times before or are they following notes/maps? What's the weather like?


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## Ravana (Feb 6, 2012)

Not to mention how high the passes are, how good and how wide the trails, what if any pack animals and provisions are available… et many cetera. Although the biggest question is also the simplest: how wide is the mountain range to begin with? 

Two things to note about Hannibal's march to Italy:
(1) His army moved _fast_, compared to most pre-mech ones. It was one of the reasons for his ongoing success. 
(2) He recruited along the way–very often the local tribes he'd just defeated. Which means that between leaving Spain and leaving the Alps, he probably lost _more_ than half his original force. (It depends on what the Roman historians were counting, though: Carthaginians, or everyone he brought with him out of the mountains. It also depends on how reliable their counts were in the first place.) Considering that in September 218, when he crossed the Rhone in southern France, his forces were still close in number to what he'd entered Gaul with, his losses in the Alps had to have been little short of catastrophic. 

A ten-mile march was considered a reasonable day's travel for most pre-mech armies–Hannibal's troops and the Roman legions being notable as exceptions, not the rule. (Apparently, Hannibal was just as surprised that the Roman army he left behind him in Gaul had materialized in front of him in Italy, as they were to learn that he'd negotiated the mountains in the same time.) That's on good terrain. Moving a large force through mountains might proceed at, oh, maybe two or three miles a day, as the bird flies: remember, mountains require that you not travel in a straight line. On climbing trails, with all the attendant switchbacks, you cover fifteen or more times the distance back and forth as you do forwards and up. (And that's a 6% grade, which is considered _steep_.) These parts only constitute a small fraction of the total trek–most of the time you'll be traveling along valleys, which are themselves probably not perfectly aligned in the direction of your desire–but they do add up. And that's before taking all the other factors into account, which can easily push the daily mileage figure well into and past the low end.

Basically, the only time anyone has ever taken large forces over mountains is when they've had no choice in the matter. Which is probably what Hannibal felt when he took to the Alps. It's a fair bet this wasn't his first choice: he moved north to avoid a Roman force that had arrived at the mouth of the Rhone, though how far he deflected his intended route of advance isn't known. (He didn't actually cross the Pyrenees, by the way: he didn't have to–he skirted around them along the Iberian coast.) Even if he had intended coming over the Alps rather than along the coast, which he probably must have to some extent or he wouldn't have been as far north as he was even before dodging that army, he had likely planned on a somewhat narrower and less rigorous section. The speed at which he crossed them was a combination of his brilliance and desperation: the Romans knew he was there, and was coming, but didn't expect him to arrive before spring… no one would be crazy enough to try the mountains with winter setting in. 

While Hannibal's exact route is not known (and is the source of lively debate among military historians), it is known where he was in September of 218 and where he reappeared two months later… which means we know that in the general region where Hannibal crossed, the Alps are around sixty miles wide. So, _including_ the time between the Rhone and the mountains (a fairly short distance, depending on just how far north he went) and the march after he was through them (another forty-ish miles down the Po Valley to the Battle of Ticinus), he _averaged_ about two miles a day… much of that not in the Alps themselves. So that can give you one approximation to work with. 

Against that, the Alps aren't just any old "mountains," either, and he went through one of the highest parts; so, as the saying goes, "your results may vary."

Keep in mind that it's much easier to take a small force through mountains than a large one. The trails don't get any wider just because you have more guys with you… but they all still have to eat. Which means that the supplies have to be able to catch up with _all_ of them _every single day_. Which, basically, doesn't happen–ever. An army marching through level terrain leaves an immense "footprint" to either side of it, as quartermasters and/or foragers fan out to gather resources. This isn't an option when fanning out equals taking a tumble down a rocky slope. Indeed, large forces will often march in parallel columns, only uniting for battle, to aid in this. As well as to ease the problem of trying to get soldiers in the rear up to the van when battle _does_ occur: do the math on how far apart the first guy and the last are for your force size… one of the reasons scouts are so important, and one of the reasons you read about opposing forces camping within sight of one another the day before, sometimes several days before, the actual battle. Barring multiple parallel passes, this too isn't an option in mountains. 

So you pretty much have to figure that not only the total but also the _percentage_ of casualties will rise as the size of the force increases; eventually, you hit a point of diminishing returns, and will actually be better off taking fewer rather than more troops. I couldn't guess precisely where that occurs, but if someone gave me the job of taking an army over mountains, and the option of starting out with 20k or 70k soldiers, I'd take the lower number in a heartbeat: not only would I probably be little worse off in ending numbers, but I'd arrive faster and with my troops in better shape. I'll take 15k healthy guys over 30k mostly starving ones any day, all else being equal. We haven't even gotten into the difference in morale you'll have between a mostly-intact and a more-than-half-dead-along-the-way army. Or the surprise value, which will be utterly lost if you have to a week or two to collect all your stragglers–and if you don't, add those to the number you've actually lost: alive but absent is no more useful than dead, sick or missing important pieces from frostbite when the swords come out.

As for other historical examples… the mountainous area that's probably seen the most military activity, as far as I'm aware, is the Caucasus region. And most of that has been south of the main range itself–what would today be eastern Turkey, northwest Iran, that vicinity. The Balkans are another region to look at, though the sizes of the forces used there have generally been smaller–and the higher ranges go a long ways toward explaining borders both historical and modern. So try those out for other possibilities.


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## sashamerideth (Feb 6, 2012)

Centerfield97 said:
			
		

> So, in my WIP i have my primary city threatened by a large, impending army travelling through a substantial mountain range to conquer it in its weakened state.
> 
> How long would a large army (20,000-70,000) complete with mammoths, provisions, etc. take to cross a mountain range?  The only historical example I can think of is Hannibal crossing the Alps.  How long was that journey, and what were the casualties like?



An army of that sort moves at the speed of plot. Six to eight months is realistic, a couple months can also work, it really depends on how long you need the army to take.


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## Butterfly (Feb 6, 2012)

I've recently seen a documentary on this!

It's here Hannibal - Rome's Worst Nightmare | eDocumentary - full online documentaries

It's 1 1/2 hours long - they hit the alps about 20mins in.


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## SeverinR (Feb 9, 2012)

I would use the best and worst case scenerios to determine somewhere in between, and how many casualties could be expected.  Traveling in the mountains in the winter can kill all in route, but there wouldn't be much of a story there.

Man walking is slow, but I am not sure of if man is the slowest, do oxen move faster?  Horses could move faster but they also have to graze if available, be fed a good fiber source if not(hay is common), this takes sometime, hay can be given after dark in evening, and before breakfast in the morning, so hay takes less time, but takes more to haul.  How much hay would a calvalry and wagon pullers eat in the time period expected?
The complete force moves as fast as the slowest component.  Up hills, wagons could be the slowest and most energy burning component. If attack is not expected, cavalry can move ahead and if possible graze the horses while the rest catch up.
Moving large amounts of men/animals is no easy task, compounded by the extreme cold.
Might look at WWII accounts of the winter movements, for the suffering and the problems they had. The grunt has not changed much through out time.
Not sure what mountains would be comparable, but cold is cold.


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## Graham Irwin (Feb 9, 2012)

I think it should take as long as your story needs it to.


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## Xanados (Feb 13, 2012)

I just finished watching a 1hr 30 min documentary on Hannibal's crossing of the Alps. It was very good. And accurate.


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