On the disease issue, most people of current military age have no immunity to smallpox, having been born after it was eradicated.

swords and spears are relatively easy to learn? Well, there is difference between head-shotting someone from 300 meters and cutting his guts open from harm reach. don't underestimate the psychological impact of such actions.

i can agree with spear its the main weapon for infantry even peasant can learn it quick, but sword... this depend i even think they may have trouble to unsheat quickly and theres also need muscle memory to make sure they dont lost their limb or other important organs and of course their life, they are far better has a chance using the dagger or grappling, even though knight and such do learn grappling technique and such and use it alot in battle especially against opponent heavy plate knight but since this is early crusade and against saracen and chainmail is heavier than plate armor.....

but that aside the soldier probably can do better using crossbow anyway rather than melee combatant. assuming the soldier dont know how to create gun or teach the technology to the crusader

just like genoese crossbowman, which exist in early crusade and become prominence unit.

I think you greatly underestimate medieval swordsmen. Even if you could spare a month or two to actually teach our soldiers basics of these weapons, there is no chance they will put up any proper fight against trained units. They will be better than random peasants which were often used for crusades - discipline, stronger and better fed - but not a match for trained people who have the same skills AND know how to use a sword.

Superior training? Not necessarily. Certain medieval swordsmen were trained extensively in hand-to-hand combat. And this wasn't limited to weapons training. Also, opposed to modern troops their combat training focused on H2H, not on marksmanship.

@L.Dutch-ReinstateMonica - Well, on the one side shooting someone on distance won't have the same impact as slashing someone in melee, but on the other hand having people around you arbitrarily being injured and die to quasi-invisible, super-speed projectiles and random explosions isn't much less traumatizing either. But if someone is a good soldier (and not drafted against his will for example), he'll have the capability to kill and to continue to function, no matter the odds.

@ZizyArcher Are you sure that the armies that got to Jerusalem included significant numbers of random peasants? Travel by sea had a big per-person cost and one would get more bang for the buck by bringing a smaller number of trained men-at-arms and knights. Travel by land weeded out all but the best supported and best equipped.

@PatriciaShanahan I think the more serious threat would be all the diseases the modern folks would be carrying that the locals have no immunity to. I suspect it'd be just like the epidemics that the colonists brought to the new world with them.

@MorrisTheCat What diseases are endemic now but unknown in medieval Europe? One of the worst of the colonization epidemics was smallpox, and that history tells us what happens when Europeans from before smallpox vaccination meet people with no vaccination or exposure to smallpox. A few of the longest-serving members of the company may have been vaccinated by the army in about 2002. Over 50's would have been vaccinated before 1972, when routine vaccination stopped in the US.

@PatriciaShanahan It's not that there are completely new diseases, it's that the ones we have have a thousand years of evolution behind them. Influenza, for starters, is very different today than it was a thousand years ago, and it's very unlikely that crusade-era Europeans would have any immunity to it. The intense use of antibiotics over the last century has resulted in lots of new strains of old diseases that are much more dangerous than they used to be. See: coronavirus.

@PatriciaShanahan In 2002 the US military began smallpox vaccinations. Anybody who has been on active duty since 2002 will have received the shot.

Coronavirus is a virus, so not affected by antibiotics. The travelers will be just as vulnerable to 12th century strains of flu as 12th century people will be to 21st century strains.

spears may be easy to learn, swords not so much. Not only do most people have very poor sense of edge alignment which is very important to the ability to cut but there is an enormous amount of nuanced movements that a trained swordsmen will have down to reflex that untrained people won't be able to think of much less execute

You're severely mistaken about the survival aspect. Modern soldiers are trained to use modern survival tools - including modern rations. From my own experience, unless you're from a very very very specialized troop you would be dead quite quickly if you try to forage the wrong plants in the medieval wilderness. Also hunting - most people nowadays can't hunt without proper weapons. Take those away, and your soldiers will starve to death.

@MorrisTheCat Coronavirus has nothing to do with antibiotics. And the overuse of antibiotics hasn't made bacteria more deadly, only more resistant to antibiotics which usually makes them less deadly.

@T.Sar They will have locals to tell them not to eat that mushroom.

@Harper-ReinstateMonica I seriously doubt that there will always be a local at hand to do that.

@T.Sar being a Q&A site, answers are in the context of questions. The question states they joined an indigenous army, end of para 1.

@Harper-ReinstateMonica I reaffirm - that doesn't mean the indigenous people are always around. A lot of situations can come up that let a "future" soldier alone in the field.

@T.Sar It's hardly a secret that foraging is a huge part of a medieval army's MO, and logistics will be front and center for the modern commanders. "How do I feed my men" isn't going to 'slip their minds'. Command should be working with the locals to collect and propagate the necessary training before they send people anywhere they'd be alone.

@Harper-ReinstateMonica Something that is obvious for a guide might not be obvious for the soldier. Most people nowadays knows that Deadly Nightshade is a very dangerous plant. Almost no-one can correctly identify one in the wild. You're seriously underestimating the amount of survival training one needs to function in that timeframe.

"we've had centuries to improve on knife-fighting skills" – and yet we haven't, because basic body mechanics haven't changed. Modern knife fighting skills are virtually identical to Marozzo's 1536 textbook.

Also, hand-to-hand combat skills which modern soldiers are trained in are largely limited to knives, either as a last-ditch, or perhaps for quiet take-downs if they're special forces. But knives are short. SAS guy with a knife will lose fairly reliably to average trained medieval pikeman.

The average soldier would be useless with a bow and arrow - and as such would likely not be of any use in an English line. They would also not know the tactics as well as other commanders to deal with such weapons - so would be of no use to the planning. They might be physically strong enough to be able to swing a sword and move in the armour - but so what? Hand to hand fighting in armour is nothing like it is without it. In short - all their training is useless.

@patricia that's what vaccines are for. Smallpox vaccine was the first one and given it could be used in bioterrorism they might already be vaccinated if they were deployed to third-world countries where the disease exists. In some countries it is mandatory. everyone has the mark in the upper arm. (This space reserved for anti-vaxx outrage).

Guns are way easier to learn to be honest, a mediocre shooter will still kill people , a mediocre swordsman will probably be beaten by a farmer. I strongly disagree with the premise that soldier were less disciplined than the ones today, you are comparing a navy seal to an army composed of farmers, you need to compare the top ones with the top ones, wich would be spartans, romans and others. Also the trainings we have today try to mimic the harshness of the ones they had back than, plenty os spartan KIDS would die while training. We have better tactics and equipments, that's all.

@GaboSampaio, instead of comparing the top to top, you should be comparing the bottom to the bottom, and even the most undisciplined military company today would be more disciplined than the worst company then, which was made up of slaves, convicts, and others who didn't want to be fighting. And tactics have a huge impact on who wins or loses.

I've tried to learn sword. Shooting is way easier. If you only try to "hack and slash" with your sword or pike, you are as good as dead, thank you.

I was in the German Army. We never learned any close combat Techniques. I also do medieval sword fighting. NO it is not easy to learn! neither is a bow. Learning to use a gun is waaay easier. That´s why firearms where widely adopted in a time where a trained longbowman was may more effective and deadly. The only thing that comes that could come in handy is guerrilla tactics and camouflage.

Also, nothing prepares you for your first "real" battle, even if it is only reenacted. The sheer pressure of a closed line of soldiers and the direct and brutal force you have to exert will get any modern soldier to the brink of exhaustion within minutes!

Going to second @Daniel (though not from firsthand experience). It's my understanding that even up to the level of elite special forces, there is very little time spent in anything beyond the basics of hand to hand combat. It is fairly rare for present-day soldiers to encounter hand to hand combat and it is much harder to master, thus the majority of time is spent gaining proficiency in things like marksmanship and small unit tactics.