My character is a modern guy, an avid swordfighter and has had a bit of basic military training. He then time travels to the Ancient Rome and joins the Roman Imperial Army. How easily could he adapt, since he’s never fought in armour and with a shield before? Would his modern techniques cause issues of integration? Also, if his swordfighting skills aren’t set for killing as he learned it as an art form, would that be an issue too?
Ironically, any combat training he’s had will be among the least useful skills he takes back with him. Basic, modern, knowledge you take for granted is far more significant. Particularly anything technical. A basic grasp of chemistry, medicine, or even metallurgy could radically alter the course of history.
So let’s start with the sword fighting. HEMA practitioners do not fight using historical techniques; they use recreations. We have the training manuals but we don’t have access to the masters themselves. Meaning there’s a huge skill drop.
In martial arts, it’s extremely important to have a trained practitioner on hand while you’re learning. They can see the mistakes you make, and correct those as you go, so you do not train them in.
In the case of HEMA, because there were no living masters, any mistakes made by the people studying initially became baked into the martial art itself.
Training in mistakes is a serious issue, and is one that can haunt a martial artist. When your muscle memory tells you to do one thing, and you’re not supposed to, it’s very hard to break that behavior. This is something that could be a serious issue for your character, though, honestly, all of their training is going to be irrelevant.
HEMA seeks to recreate a fighting style that saw use in European warfare. The modern use is recreational (or educational, if you prefer), it’s not intended for actual battlefield usage.
If a HEMA practitioner is honest with themselves, they’ll admit that they would not stand a chance against actual soldiers from the timeframe they’re recreating. Their training just isn’t good enough to keep up with people who leaned this stuff to avoid death.
Beyond that, HEMA is still sampling from a specific timeframe. One which, for the most part, does not overlap with Imperial Rome. There are Roman Legion reenactments. It is possible your character did that. But, when you’re talking about “an avid sword fighter,” that’s either someone who follows either Italian or German school fencing. In either case, you’re talking about training with a weapon that won’t exist for, at least, another thousand years.
One thing your character may have in spades is a level of strategic skill that is uncommon or impossible to replicate historically. This is due two things, first, if they have a background in military history (even if it’s just as a hobbyist) they’ll have extensive knowledge over what’s been tried and worked, or hasn’t.
In some cases, they may even have a pretty good read on who their facing.
Another hobby that can pay dividends is strategy games. Now, there’s nothing new about the idea of wargaming. Chess is a wargame. But, the level of sophistication, and the variety of potential scenarios has increased dramatically over time. The war games today are far more instructive on commanding a large force than historical games would have been.
None of this matters if your character isn’t in a command position (and they probably wouldn’t), but it’s worth remembering.
Basic medical knowledge, the kind you passively pick up, living in the 21st century, has numerous, significant, advantages over someone living in the first century AD. For one thing, you know to disinfect a wound. You know you can use clear alchohol to do that. And you understand that if you don’t, the wound could become infected. You also probably know you could boil bandages to kill anything on them (even if we don’t do this today, because bandages are usually disposable), and that you should change the bandages out for clean fairly regularly. All of this to prevent bacterial infection, because that will kill you.
Your average Roman Legionnaire did not know this. Your average soldier in the mid-19th did not know this.
Modern wound care, something so basic, you’ve probably learned about this from entertainment, is an enormous technological advancement over what the characters in the past would know.
If your character has an actual medical background, (a doctor, a nurse, an EMT, even just a veterinarian), they have just become the most skilled medical practitioner in the world. The information they have is literally thousands of years more advanced than anyone else. This is far more valuable than their ability to swing a sword.
If your character has background in chemistry, buckle up. You can synthesize black powder using a mixture of carbon (so, charcoal will work), sulfur, and sodium nitrate (saltpeter). You’ll need to work a bit on getting higher quality metals, but that’s not much of an ask for a chemist. Congratulations, your time traveler just invented guns using reasonably available materials. They aren’t particularly good guns, but a bullet’s a bullet.
I’m also going to point out, for someone with a background in chemistry, this is one of the least disruptive things they can do.
If you took chemistry in high school, you probably made a potato battery. There’s a lot of ways you can generate electricity if you know it’s a thing, and want to do something with it. You can make liquid batteries that can be refilled. Now, if you’re living in the first century, this is a big, “so what?” You wouldn’t know what you could do with the stuff. For someone with a modern background? You know what you can do with electricity. It’s easy to think, oh lights, but, if you understand how the components are put together, you might be able to construct something like non-portable radios. Sure, you can’t actually talk through them, but that’s why things like Morse Code exist.
Metallurgy is another one that can get downright nuts. If your character knows how to make crucible steel, and understands basic, modern, forging techniques, they’re going to be able to make weapons that are without peer in the past. Sure, it’s not guns, but being able to take, even low quality, modern steel blades into combat against foes equipped with bronze and iron? That’s not going to end well for their enemies.
When you’re dealing with time travel, your character’s combat prowess is one of the least useful assets they have. Their weapons (if they brought any), are more significant, but your character’s technical knowledge is real advantage here.
I know I focused on it, but in many cases, it’s not even, really, the combat applications for skill sets. Their non-combat skills are immensely more valuable to the civilization they just landed in. Hell, even just a modern understanding of economics would be world changing for a merchant in the first century.
It’s easy to look at what you know, and think that this stuff is obvious, and everyone must have known this. Truth is, we stand on the shoulders of giants. The world we live in today, the knowledge we have today, is the product of millennia of advancement. Fold that over, send some of that information back, and everything changes.