# How could a time traveller change medieval history using only modern knowledge? [duplicate]

How could a time traveller from our era change the outcome of a medieval battle using only knowledge of history, geography and modern science?

Lets suppose our time traveller is a learned person with a good grasp of at least high school science and history.

He / she has the ear of the king and has prep time of up to six months to a year before a decisive battle is to be fought against a foe with superior numbers, whom history decrees will win the battle

Chemistry: Gunpowder was my first thought, but getting access to base ingredients would be difficult.

Physics: Better siege weapons, perhaps more effective trebuchets, but these won't help with a confrontation between two armies on the field

How does our traveller make a difference?

## marked as duplicate by Zxyrra, Frostfyre, Youstay Igo, EveryBitHelps, AzuaronFeb 21 '17 at 19:32

• What type of battle is it going to be? Is it terrestrial or naval? Also, which battle in history is it that you want to change? There are many specifics about every historical battle which we know now and which could turn the outcome. A general answer can be written too, but specific details would help a lot narrow down to particulars which are the most important for that battle. – Youstay Igo Feb 21 '17 at 8:24
• If you're talking about Europe, gunpowder was known since the 12th century CE and was used in war by the 1400s – nzaman Feb 21 '17 at 8:28
• Oddly similar to What could an average modern human achieve in medieval times? – Theraot Feb 21 '17 at 8:31
• See Mark Twain's "A Connecticut Yankee in King Arthur's Court", which is exactly this – pjc50 Feb 21 '17 at 10:23
• @pjc50, to be fair, he had a farmers almanac, the Wikipedia of Twain's generation – user1717828 Feb 21 '17 at 13:22

Trying to implement modern tehnology in a short time is a non-starter. Don't go that way. Use the local tehnology and military knowledge augmented with the unique intelligence opportunity brought by the special advisor.

They are a time traveller, right? So they know (1) how large is the opposing force and its composition, (2) when it will be assembled and where, (3) what route they will take and (4) where the battle is to be fought. Using this knowledge they can instruct the king to:

• Assemble a larger force. Medieval armies were tiny. Any half decent kingdom could easily assemble a larger force than what the enemy thought was enough -- in medieval times the real problem was the abysmal logistics, so, if the king knows precisely when and were to assemble to host it should be easily feasible to achieve superior numbers.

• Counteract the composition of the opposing force. The enemy comes with heavy cavalry? Prepare the field with caltrops, masked ditches, and other anti-cavalry surprises. If they have a little time they could introduce effective anti-cavalry tactics, such as pike formations. The enemy relies on Genoese mercenaries armed with cross-bows? Fight behind field fortifications. Surely the generals will know what to do if you tell them in advance what the enemy will bring.

• Choose a better battlefield. The time traveller knows the route and the schedule of the opposing force: the generals of the king can exploit this knowledge to choose a battlefield which favors them.

• By knowing the size, composition, route and schedule of the opposing force your side can easily achieve strategic surprise; if they can't they amply deserve to lose. Launch a pre-emptive attack. Attach the opposing force en route. Defeat the opponents in detail before they can concentrate their forces.

• Surely time travel will invalidate any events happened between those two dates. The history will swiftly diverge from the learned history and this method may work at most in one battle. Finally, I don't believe the time traveller will remember details of every battle. He/she should know which battle it would be before time travelling in order any of those to happen. – Cem Kalyoncu Feb 21 '17 at 11:11
• I think that if something goes differently the spies and recon will send the different information, which leads to different decisions. So as soon as there is something different, the battle will be significantly different. – user3644640 Feb 21 '17 at 11:15
• @Cem Kalyoncu: At the moment, OP wants to "change the outcome of a medieval battle". – Edheldil Feb 21 '17 at 12:41
• "By the way.... make sure your soldiers wash properly, make sure to dig proper latrines in camps, invest in some soap". Disease killed a surprising number of soldiers. basic sanitation steps, enforced hard would help quite a bit. – Murphy Feb 21 '17 at 13:24
• @Murphy: Depends on what specific army we are talking about. Don't believe everything you see on TV about the Middle Ages -- it was a long time, and it covers a lot of very different nations and armies. Some medieval armies were very unprofessional and dirty; some were very well trained and did dig proper latrines. Not everybody who lived during the European Middle Ages was an illiterate north-west European Christian barbarian: for example, the cataphract cavalry of the (Eastern) Roman Empire was the very model of "a well-regulated militia". – AlexP Feb 21 '17 at 13:37

The first and most important thing to teach, and one that also can be applied in the available time, is hygiene, as mentioned by others. I remember reading that in medieval times soldiers often weren't allowed to remove their uniform at any time. Dump that stupid rule and make them wash. Also, make them wash their uniforms. Aside from other things it will also mightily impress the opposing army.
Once you got them started on hygiene, make sure they understand that latrines are to be built away from water sources.

Next, teach them about logistics: Make sure your own troops are well provisioned. And focus your military attention on disrupting the enemies supply chain, whenever possible by taking their food for yourself.
And teach them that pillaging your own farms is really a stupid idea.

Then, discuss tactics. Training infantry was not horribly difficult, but it's still a good idea to make sure that you have as many battleworthy troops after a battle as possible: There's likely to be a next battle, and once the enemy notices that your army is nearly as strong as before, it will diminish their morale. And their troops... And after the war there are fields to tend to, which is also easier when you haven't killed all your labour force.
So, teach them not to brute-force battles, but to employ hit-and-run-tactics. Teach them that killing the enemy commanders is much more efficient than facing an entire army up front.

In short: Avoid battle where possible, and never let the enemy dictate the playground, or the rules, and most of all, never both.

And after the battle, wash. Use soap. pillage the battlefield and burn the dead. And wash again.

• If you start killing the enemy commanders, everyone is going to take war to you. Medieval warfare had its rules, and disregarding them so openly would have serious repercussions. A lot of what you mention was understood, but impractical - you'd need well trained troops, or way more resources than you had at hand. It's easy to write "make sure your own troops are well provisioned", but how? Soap was used (though rather expensive), many militaries understood how latrines are built, pillaging your own farms was effective (for both your logistics and denying them to enemy logistics). – Luaan Feb 21 '17 at 14:00
• For some reason, commanders liked the rule about not killing commanders. – Konerak Feb 21 '17 at 18:04
• Depending on the army, battle tactics were excellent at the time. An average Joe with high school knowledge cannot best Alexander, Genghis Khan, Hannibal or the other well-known generals. – Cem Kalyoncu Feb 21 '17 at 18:14
• H Beam Piper's Lord Kalvan seems relevant... – DJohnM Feb 21 '17 at 21:06
• The "use soap and tactics" approach is great. I'd like to add the concept of camouflaged clothes came very late in history. Having a force not suffering from dysentery, which doesn't try to scare the enemy by numbers, looks and yelling (but it's performance) might have been be a great difference! – jvb Jun 23 '17 at 6:39

Communications. Semaphores.

In ancient times it could take weeks to get a message from one side of the country to the other. The semaphore first appeared in Europe in the 1790s, and suddenly commanders could send and receive data within minutes. This gave them a huge advantage.

It's easy to build one with low tech, with flags and levers on a hilltop, or to do it manually (I learned to do this in the Brownies). The hardest part would be disseminating the code. You would need your own code to stop competitors reading it.

edit: add to that, send messages at night using blink code and a covered lantern. You don't need a lot of text to send standard military messages, you just need a code worked out beforehand, and change it regularly. The covered lantern has the advantage of being directional, instead of the semaphore on a hilltop which can be seen by everyone.

• Excellent! +1. Talking about the logistics, you have other ideas for quick and detailed communication? – Youstay Igo Feb 21 '17 at 17:59

Some ideas:

Steam machines: Are not that difficult to build for an experienced blacksmith. You may get ironclads and other steam machines.

Distiled Alcohol: It's highly flammable a not so difficult to get from wine or other alcoholic beverages. You can create plenty of machines being fueled by alcohol.

Optics: Not that difficult if you have people who know how to make glass works. You can get binoculars and lenses.

Hot Air Balloons: Not difficult to make. They would give archers some advantage and would frighten your enemies.

Compressed Air Cannons and Catapults: Once you know how to build valves and gears is not that difficult.

Methane: You can get this powerful gas from animal faeces and decomposition.

• +1 for the hab idea. Didn't think about that. Truly marvelous! – Youstay Igo Feb 21 '17 at 9:15
• Actually, optics and steam machines require modern tools and materials to do properly. Maybe you could create lenses with medieval tech, but it would take very long time. Steam machines wouldn't be very efficient without high precision and high quality steel. – Lope Feb 21 '17 at 10:34
• @PbxMan I am familiar with both examples, but that doesn't contradict what I said. It is one thing to make something move using steam and something completely else to be able to extract useful amount of work out of the steam powered system. Former is a practically useless toy, latter requires much more advanced engineering. Similarly for telescopes, being familiar with optical principles does not automatically allow you to build lenses with high enough quality – Lope Feb 21 '17 at 13:10
• Distilled alcohol: maybe. Hot Air Balloons: maybe. textiles were expensive as hell, you may not realise how expensive but it would make the cost of a hot air balloon very very high. For context a commoners set of clothes would have been on a par with a car in terms of relative cost. Distillation though would provide a trade good and might improve your kingdoms financial state coming up to the war. – Murphy Feb 21 '17 at 13:19
• @Daerdemandt If you think you can build a steam-powered loom from memory without modern tools good luck to you. – Murphy Feb 21 '17 at 15:14

This is kind of broad, but anybody with sufficient modern scientific knowledge and modern combat tactics can potentially determine the outcome of any battle in the medieval times.

While the possibilities are almost endless, here are some which I thought would be the easiest and the most effective.

1- Use historical knowledge and battle specifics

Someone from our times would know the details about the particular battle and how the armies were arranged, which weapons were used and what were the strengths and weaknesses of each side. With this hindsight knowledge, you can easily overcome your party's weaknesses and focus on the weaknesses of your foe. Knowledge is power, and future knowledge is like nuclear power!

2- Utilize biochemical weapons to decimate and demoralize the enemy

You have lots and lots and lots of options here. Coat your arrows and spears with venom collected from snakes, scorpions, spiders, poison dart frogs (central and South America only) and hornets. For mineral toxins to coat your blades, use corrosive sublimate (HgCl$_2$), phosphorous salts and sulfide salts. In plant poisons, you can use oleander, calotropis, digitalis, bushman's poison, strychnos, datura, rosary pea, wolfsbane, hemlock ... (and a couple dozen others. I think the point has been made)

3- Utilize wind!

Before the battle, when the armies are camped in front of each other, if the wind is blowing from your side to them, burn noxious chemicals so that the fumes are blown toward your enemy, causing damage and panic. Large torches, coated with a mixture of crude oil, sulfur and powdered oleander will do the job nicely. The fumes will be acidic and toxic, not only inducing cough and teary eyes, but also causing panic and decrease the morale of the enemy.

4- Use blinding beam to disorient your enemy

You would need many mirrors of 1 square foot size. Install the army of these mirror holders (about 300 of these will do) at some out-of-reach location (e.g. at the top of a very steep hill). They will focus the reflected sunlight beam at the enemy's soldiers (specifically targetting archers and cavalry). While they will not be able to set anyone on fire or inflict severe burns, they will be able to blind them and create panic and disorientation, helping your side.

5- Implant poison coated punji sticks in the way of your enemy

I mean these.

Considering that you (the time traveller) in hindsight, knows which path the enemy will take to the battlefield, you can easily deploy tens of thousands of these pointed sticks with poisoned tips, in his way. You can plant them in the jungles and woods in his path and you can lay punji traps for him, by digging up pits, planting punji sticks in them and then covering up the top with a thin layer of grass and sticks. Given a couple weeks, the upper layer would completely naturalize with the earth, making it impossible to see the trap.

The results would be simply disastrous for the enemy.

6- Use caltrops to disable cavalry

These are caltrops.

Prepare lots of these and give at least a couple dozen to each foot soldier and cavalryman in your army. Utilize them to disable fast cavalry attacks during the battle. Simply throw the caltrops in the way of the invading cavalry and many of them will never reach you at all. It will also slow down the approach of the remainders, as they will have to watch out for these deadly spikes.

Final Word

There are many, many more methods you can deploy, but I thought these would be the easiest and most potent ones. The medic improvement and steam powered catapults (which I don't think can be built during a 6 months time) have already been mentioned by other users, so I do not need to reiterate any of those.

• Using poisonous weapons against nobles might lead to excommunication and also loss of honor, so the king might decide not to heed your advice and charge you with witchcraft or st. like that :-). – Edheldil Feb 21 '17 at 12:47
• Except point 1, none of these sound like they require modern knowledge. If they are feasible, they will already be employed by the medieval people. – ths Feb 21 '17 at 13:19
• Haha. That is like saying producing electricity was a feasible idea, Romans and Egyptians would have electric lamps in their palaces. The ideas mentioned in the answer do require modern knowledge, but not modern technology, that is why they will prove potent. Or do you think the time traveller should have tried to invent something like a nuclear bomb or a gattling gun or maxim canons? @ths – Youstay Igo Feb 21 '17 at 13:42
• Burning something nasty while wind blows to your enemy does not require modern knowledge. Making sure wind does not change in 10 seconds, on the other hand... – Daerdemandt Feb 21 '17 at 14:05
• no, but poisons or smoke are really basic. the mirror thing was done by Archimedes. Caltrops:"According to Quintus Curtius (IV.13.36), iron caltrops were used as early as 331 BC by Darius III against Alexander the Great at the Battle of Gaugamela in Persia." – ths Feb 21 '17 at 14:06

# Pasteurization

Medieval logistics were a mess. If you introduce foods that can be mass-produced and stored for weeks or even months then your logistics would be simplified a lot, which makes your armies A LOT more effective.

Just make sure people are aware of scurvy and eat their sauerkraut diligently.

Also, screw famines.

# Better looms

Introduce flying shuttle and mechanise your textile production. You can rake GDP-level lots of money this way if you can scale your production.

If you leverage those, you'd have both military and economic advantage.

Modern medicine will help a lot, reducing the number of people dying from wounds. I am talking about knowing germs exists and being able to cultivate penicillin, with that you could reduce the number of deaths greatly. Hygiene will keep your soldiers free of illnesses while knowing the importance of vitamins (found around renaissance era) will keep them strong.

Steam power will also help to power catapults and similar siege weaponry. Increasing their power and reducing the number of people manning them.

• I agree about medicine. But steam powered catapults? No way! It will take a full fledge engineer to design a steam powered trebutchet or catapult. – Youstay Igo Feb 21 '17 at 8:25
• You are completely right, but knowing steam can power it, a medieval engineer can incorporate it into its design (not average Joe who brought that knowledge). – Cem Kalyoncu Feb 21 '17 at 8:26
• Even if you can't cultivate penicillin, you still might be able to make sulfa powder. – A. I. Breveleri Feb 21 '17 at 11:22

Politics.

If you came up of a really good invention - you will still have to build it using the medieval materials, and then produce it in big numbers, and then train soldiers to use it.
If you only have a year, you probably don't have the time for that.

But you got the ear of the king (umm, I hope not literally), so you can at least win more time, or maybe even avoid the var.
You probably know the situation in the opposing kingdom. Maybe you'll be able to find a common enemy, or to spark a rebellion there, or to provoke it to attack some other country first. Maybe try to appear more threatening (showing off steam engines?), or more useful as an ally (penicillin trade pact?).

But in the meantime, yes, keep improving mining, medicine and hygiene. You probably will be attacked by someone else, anyway.

So, there are a few major "any modern person would know about these" things that your time traveler could convey. They wouldn't necessarily win the battle, but they would improve the overall conditions of your army and thereby increase your odds:

• Basic hygiene: Wash your hands and faces, and ideally the rest of yourself, and use soap! Don't eat rotten/old food! Diseases are spread by germs, which can be spread by body fluids or contact (though different diseases use different methods), so cover your mouth when you sneeze or cough!
• Medical hygiene: use distilled alcohol to clean wounds before binding them to prevent sepsis! Clean tools and surfaces with distilled alcohol or dilute bleach before using them!
• Lead & Mercury, dangers thereof: That stuff's poisonous! Don't make cups and plates out of it or use it for plumbing! And stop trying to use Mercury as a medicine for anything!

You specified that your time traveler knows high-school chemistry. If they remember the medieval names of various chemicals, then they can advance chemistry by a few hundred years by teaching the kingdom's alchemists all sorts of interesting combinations. The ones pertaining to warfare mostly involv aqua fortis (aka nitric acid) and vitriol (aka sulfuric acid).

One low-hanging combination is aqua fortis + cotton (+ vitriol optionally) = guncotton. It's like gunpowder, but more powerful! Keep it dampened with dilute alcohol, though; it's very sensitive. Use it to fill artillery shells or grenade-type bombs for increased explosive power. With a some (rather explosive) trial-and-error, they might also be able to use it to make smokeless powders for their firearms. This does depend a lot on the strengths of the acids used, though, so it may still be beyond the resources you have available.

As @nzaman mentioned in their comment, gunpowder was already well-known to the medieval world. However, early firearms were finicky and unreliable; it wasn't really until the development of the flintlock musket in the mid-1600s that they really came into their own. So if your time traveler is familiar with how flintlocks work, then they've got a head-start there, too. But there's a simpler firearm-related invention that can give your side a distinct advantage ahead of its time: the Minié ball.

Rifles were known as early as the late 1400s, but early rifles were unreliable and slow to load, since the lead ball had to be hammered into the barrel to engage with the rifling of the barrel, and the gunpowder quickly fouled the rifling. The Minié ball revolutionized gunpowder warfare by letting the bullet expand to grip the rifling when fired, making it as quick to load as a smoothbore musket. Also, the conical shape and better seal against the barrel gave a higher velocity, and thus better penetrating power. Medieval armor was frequently "proofed" against bullets, but only the low-powered, short-range weapons available at the time, not modern rifle bullets. If your traveler still has problems, they could experiment with adding a hardened steel core to the bullet as a penetrator, but that's probably overkill.

So now your force has accurate, high-velocity rifle fire from significantly longer ranges than the enemy is expecting. The same methods scale up for cannons, improving their range and accuracy as well.

Also, the time traveler should also pre-invent the bayonet, so that their army doesn't need dedicated pikemen to protect against melee attacks or cavalry charges.

Now they are effectively bringing a mid-1800s army to a medieval fight. Have fun!

• Lots of countries have had civil wars. Which particular era are you talking about in the last paragraph? – Peter Taylor Feb 21 '17 at 16:55
• @PeterTaylor - Good catch, and my apologies. Edited! – Salda007 Feb 21 '17 at 18:43

Depending on how technically savvy this person is, the time traveler would actually be able to create a stream engine and kick start the industrial revolution by simply introducing the people to this technology. He could also introduce electricity and make generators driven by the previously introduced steam engine. With enhanced industry the production of armor and weapons can increase granted they manage to set up production soon enough.

With the introduction of the steam engine new ways to conduct agriculture also emerge. With this new technology they can easily establish a way more efficient resource production and likely higher quality armor and weapons, thus overwhelming the opponent.

On an even longer time, with the technological kickstart the kingdom received it will soon surpass technology and wealth of every bordering kingdom.

• Steam engine will give you nothing if you have a year. You would need to introduce them to better metallurgy and mining. Mass produced armour and swords would give you nothing if would be poor quality steel (because they can't mine more iron they add more coal). Also assuming that OP mentioned they don't have enough people not no enough weapons. – SZCZERZO KŁY Feb 21 '17 at 8:34

Let's assume our TT have no knowledge of this battle so he can't prepare and predicts for particular moves.
So chemistry - gunpowder is safe and easy choice, there is such things as "difficult ingredients", it's just coal, sulphur and potassium nitrate that had to be imported from India anyway. Then comes to mind hydrochloric acid or sulphur acid. You don't need much to have a great psychological impact on the enemy. Second one is mustard gas (because if you have hydrochloric acid you can make mustard gas).
But that's just dirty tricks. If you want to play clean then:

• better mining equipment for trenches and pits
• entanglements
• artillery (balistas made with modern physics and more efficient ammo)
• Smaller army means better equipment for fewer people is needed, better armour with more movability, better training and more diverse use of cavalry.

Well in my opinion, I do not think big inventions are needed to win battles of the middle ages, rather knowledge and strategy.

In the first place the armies of that time, were usually disorganized and formed in its majority by units of cavalry (usually heavy cavalry), light infatneria and archers.

Having full knowledge of the strategies used in antiquity, it is not difficult to organize an army difficult to defeat. Based on knowledge of minum complexity:

• Better use of pikes:

Units such as the Landsknecht, the Swiss Pikemen or the Spanish Third developed very advanced and effective tactics in combat with the correct use of the pikes, was such the effectiveness of these units that came to be used in the conquest of America.

The tactics of these units were very effective against the heavy cavalry, main force and power of the armies during the age.

• Use of repetitive crossbows:

These crossbows already existed at this time, but were not widely used due to various design problems, which with a couple of current improvements (simple improvements) could turn into terrible weapons on the battlefields.

• Use the cavalry, but do not depend on it.

Cavalry was a very expensive unit during the middle ages, especially heavy cavalry. So losing these units meant a higher cost than other units, so you could use a smaller amount.

Having made the choice of the previous army, I developed the strategy to follow.

The combination is as follows, compact groups of pikemen with large pikes with at least two rows of pikes, on the third row crossbowmen and groups of heavy and light cavalry on the flanks.

Using the tactic developed by Alexander the Greater, "the hammer and the anvil", but now we will have a better cavalry to attack the flanks and much better lines of infantry protected by crossbowmen in case of attempts of attacks of the rival cavalry.

In addition most of the armies of the middle age that depended on the cavalry to win the battles, could not do it since to throw the cavalry against our lines of piqueros would be a suicide

• Not depending on heavy cavalry could alienate the noblemen, and lose their support, though. – Edheldil Feb 21 '17 at 12:38
• I do not understand your comment, because you should lose the support of the nobility? If you win all the battles without any opposition and rewards those nobles, do you really think they will worry about not being in the center of the battle? – Gawey Feb 21 '17 at 12:51
• Well, yes. They are nobles because of their feudal contract. If you destroy that role, they lose most of their usefulness and power. You let the rabble do the real fighting, and it becomes very obvious that many of the nobles aren't all that useful or desirable :) This has happened multiple times in Europe's history, and was being prevented by force most of the time as well (prohibiting common people and burgeois from "nobly" things). You'd be introducing a major cultural shift, threatening enough to get the nobles conspiring against you or their liege. – Luaan Feb 21 '17 at 14:09
• I think it's not a problem that can not be solved ... If you want to win battles, my strategy is the logical evolution regarding the military issue. Some possible solutions would be to increase the rewards by conquest, to make them believe that it is the best strategy or even in case of rebellion apply the same strategy on them, surely only use cavalry and squires ... – Gawey Feb 22 '17 at 10:10

Generate electricity using waterfalls and so on just to make some electric fence. You don't need a precise, steady voltage/amperage, you just need to electrify some wires/gates as specific points and that will stop many troops. Great for defence.

• Using crude wires and relying on cloth for isolation? I think this smells more like a way to kill your own men... A Wimshurst machine might be an option. It probably won't kill, but it might at least temporarily repel attackers. However, with a good show, like demonstrating the "magic lightning machine" when the weather conditions make it likely for a real thunderstorm to appear, might convince the enemy that your magic is strong enough to cause them trouble, thereby causing a "victory by diplomatic means". Or your assassination. – Klaws Feb 22 '17 at 10:48
• No, a simple fence, with your wires standing in wood poles, enough to hurt a bit the first attackers. Won't stop everybody but will give some surprise advantage to your troop. – woliveirajr Feb 22 '17 at 20:25