Coming up with an acceptable definition for what is and isn’t a katana is a lot easier than it is for the longsword, because, unlike the longsword, the katana has a fairly standardized form and function throughout much of its history. That’s not to say that there wasn’t variation; technology, tactics, and fashion affected the katana as much as they did any other weapon throughout history. Blades shortened and lengthened, curvatures increased and diminished, and the balance between functional and decorative swung back and forth from times of war to times of peace. 

Generally speaking, though, katanas can be defined as a type of Japanese sword with a curved blade sharpened along a single edge, typically between 60–80 cm (23.62–31.5 in) in length and weight between 1-1.5 kg (2.2 to 3.3 pounds.) The hilts were designed for use with two hands, which were protected by an abbreviated circular or rectangular guard known as the tsuba. 

Despite the two-handed grip, the katana bears a much stronger resemblance to Western sabers than it does the longsword, and shares many of their strengths and weaknesses. 

Where the blade of the katana truly excels is cutting. Among enthusiasts, it’s considered one of the finest cutting blades in history, and tests have shown that it can quite effectively lop off limbs and heads. Much like similarly competent sabers, though, the curved blade presents some difficulty when it comes to stabbing; since the point is not in line with the hilt, it can be difficult to index properly for a stab, and is less efficient than an inline design for delivering one’s strength to the blow. 

Unlike most sabers (and also the longsword), the blade of the katana has surprisingly little flex to it. If you’re a fan of context clues, you might notice that the weight is broadly similar to the longsword, but the overall length of the blade is much shorter. You might surmise that this means the blade is probably thicker, and you’d be correct. 

Katana blades tend to be rather thick along the spines compared to many European swords, and the cross sectional geometry tends to be much simpler as well. Although katanas may or may not have a fuller [fuller (n): commonly and incorrectly referred to as a blood groove, the fuller is a rounded or beveled longitudinal groove that helps reduce weight and increase rigidity], the average blade tended to have a relatively simple but robust chisel profile. There are a number of reasons as to why (more on that later), but the end result is a blade that doesn’t like to bend and flex. 

If you want a blade that’s exceptionally good at cutting, this is all really good news. 

And make no mistake, the katana really is an exceptional cutter. The blade doesn’t have much flex, so it’s less likely to disperse energy along its length and impart more to the target. The chisel-like bevel is heavier than a European-style hollow grind, but it’s more efficient at parting through material. The two handed grip is excellent for delivering maximum power while retaining a great deal of control. And, while we haven’t covered the differential hardening process yet, the end result is a blade that can be honed to a much finer edge than you typically would a through-tempered blade. 

There are blades that can keep up with the katana in the cut, mind. Most of them are sabers of some sort; the Turkish kilij comes to mind. But, none of those blades have the sheer pop culture presence of the katana. In movies, TV shows, anime, videogames, and our hearts, the katana is almost mystically sharp, capable of slicing through bullets, the guns that fired them, and the tank that the guy who fired the gun was riding in. A katana should be able to pierce through the heavens and, if you meet god, a katana will cut him down. 

That’s all well and good if you’re a lead writer on an anime, but it is generally ill advised to swing swords at tanks in the real world. For that matter, I can’t really recommend swinging a katana at anything metal, because that's an excellent way to mess up your blade.

There are folks who claim that the katana was only ever a sidearm used in personal duels, and not a battlefield weapon. Those folks will go on to claim that it was never used against anyone in armor, and in fact, was incapable of dealing with armored foes. 

The first part of that is just as bullshit as the idea that the katana can cut through the side of an SUV without difficulty. To say a katana couldn’t be or was never pressed into service as a battlefield weapon is just asinine. There are fairly credible claims that a guy in Iraq got a kill with a plastic spoon. Soldiers will use just about anything to shank a motherfucker in a pinch, and a sword is a pretty good candidate for the job. 

Plus, there are historical sources that depict the katana and other Japanese swords being used in battle. Yes, it’s true that, depending on the era, bows and polearms were considered a samurai’s primary weapon. It’s also true that the katana was in use for something like 700 years. Japan had a lot of fighting over the centuries, and both the yumi and naginata fell out of favor as time marched on and guns became more popular. 

You know what never fell out of favor? Shanking a motherfucker with a katana. 

Aside: there’s this idea that samurai never used guns, and it’s as factually incorrect as the idea that a katana could cut through one. The Japanese had a whole-ass martial art (Hōjutsu) for the damn things. Granted, as near as I can tell it was mostly for the poors, but that’s not to say that the samurai themselves never got in on the act. 

The bit about the katana not being able to deal with armored foes, that comes a little closer to the truth. 

I wouldn’t say the katana was incapable of dealing with armor, anymore than I would say the longsword is incapable of cutting. But, much like the longsword and cutting, it’s really not the best tool for the job. When it comes to killing someone in armor, the katana has the deck stacked against it in a lot of very important ways. 

Maybe the most important is the way the blade is made. 

If you’re even passingly familiar with weeb lore, you’ll know that the katana is made out of folded steel. I can’t be arsed to look up the copypasta about being folded a million times or whatever, but you know the one. The idea was that Japanese smiths folded their steel over and over again to increase the quality, which led to a vastly superior quality of steel over the longsword. 

This is another one of those myths with an element of truth to it. Japanese swordsmiths did, in fact, fold the steel over on itself while forming the billets that would make the blade. There was nothing particularly mystical about the process or why they did it, though. They were just making the best use of shitty raw materials. 

Fair warning: I’m going to start hurting some feelings here. There’s a lot of bullshit surrounding Japanese steel, and we can’t discuss how or why the katana ended up the way it did without shoveling the shit out of the way. If you’re absolutely married to the idea that Japanese steel is the absolute best, look away now. 

The highest quality steel used to make Japanese swords, known as tamahagane, was chock full of impurities. The smelting process involves heating iron sand in a special clay furnace until it reaches working temperature. Over the course of 1-3 days, charcoal and iron sand are combined to form the tamahagane. It usually takes several skilled craftsman working together more or less constantly to get to that point, but if all goes well, they’ll end up with a bloom of steel that weighs about two tons, give or take.

If you know anything about metallurgy, you might be somewhat horrified at this point. Bloom steel is, to put it mildly, not great steel. There’s no way to get a consistent carbon content throughout the bloom, and since it never fully liquifies, impurities remain trapped within the metal. What’s more, it’s a royal pain in the ass to work with. The bloom has a distinctly sponge-like structure riddled with air pockets, so the smith has their work cut out for them just getting the damn thing to weld to itself to form a billet. 

But, as I said earlier: the Japanese swordsmiths had ways to make it usable, folding chief among them. 

By folding the billets as they hammer the bloom, they can force the impurities, which have a different density from the steel surrounding them, to coalesce into manageable chunks. Those chunks can then be worked out. Any impurities that remain are more likely to be evenly distributed throughout the crystalline structure of the steel, reducing the chances of a fatal weak spot forming. 

But what about managing carbon content?

Well, the swordsmiths had a trick for that, too. By judging how shiny any particular lump of tamahagane is, they can get a good estimation of the carbon content. The shinier the piece, the higher the carbon, and the harder the steel can potentially end up. It’s unlikely the traditional smiths would have understood anything about carbon content or molecular structures, but they certainly would have known that shiny=hard and took it from there. 

The tamahagane bloom would typically have the hardest parts around the edge, where the bloom was exposed to oxygen that it could co-opt for the carbonization process. That would go on to form the edge of the blade, while the metal with the lower carbon content would be used elsewhere within the blade. 

See, that was another trick the makers of the katana used to get the most out of the material they had. While cheaply made blades would be made out of homogeneous billets, more skilled smiths would create laminated billets with varying layers of hardness throughout. 

Common sense might suggest that you’d want to make the whole blade as hard as possible, but reality is a little more complicated. 

High carbon steel with a high degree of hardness tends to be very brittle. This is especially true if your starting material is bloom steel with a high ratio of undesirable impurities. If it’s not handled carefully, it’s prone to chipping and shattering. But, you can get it hella sharp, and it’ll hold that edge extremely well. 

The softer steel, on the other hand, can’t hold an edge all that well. You can beat the hell out of it, though, and it probably won’t grenade on you, even with the impurities. 

Thus, katanas were typically constructed with the structural parts, such as the spine, made out of the softer steel, while the edge was made of the hard, high carbon stuff. In theory, the soft spine helped the blade absorb impacts better, allowing the smith to harden the blade far more than they might normally, so as to get a much tougher edge. 

To further enhance these qualities, and to give the blade its characteristic curve, the smith would use a process known as differential hardening. To make a long story very short, they would paint the spine of the blade with a slurry of water, clay, and sometimes ash or rust, before putting the blade in the furnace for its heat treat. When the blade is quenched, the edge cools rapidly and hardens, while the spine cools more slowly and remains softer. The differences in temperature give the blade both its distinctive curve, as well as the flowing hamon from where the clay was painted on the blade. 

So now that you’ve got that out of the way, what does any of this have to do with the katana’s ability to face armor? Because the samurai absolutely wore armor. Pretty good shit, too, comparable to its European counterparts in many respects.

The hardness of the edge presents us with our first serious problem. You may recall that harder steel is brittle steel, and less impact resistant. While the softer spine is said to give the katana’s blade a measure of shock absorption when it hits something hard, there’s only so much it can do if you go swinging at someone wearing metal armor. Hit something hard enough and you can very easily damage the cutting edge, leaving chips and cracks that can’t be easily removed. 

Yeah, I know. Anime and copypastas told you that a katana could cut through a dude wearing armor in a single swing. They lied. 

As was mentioned in the longsword section, you’re just not going to cut through reasonably well made metal armor with a slash. You need to get your point in there and go to town looking for weak points if you want to be really effective. 

What’s that, you say? We already went over how katanas aren’t that great at stabbing? Hah. You’ll be telling me little Timmy fell down the well next. 

Wait. Where’s Timmy? Has anyone seen-?

You know what? Nevermind. He was a little shit anyway. 

But let's say you get your katana in a weak point and it’s time to start sawing and prying to get to the juicy meat inside the shell. Congratulations! You’ve probably just lost your blade. 

Katanas are built rigidly for a reason. Tamahagane is not spring steel. It does not like to flex and return to its previous shape. It likes to bend, and it likes to snap. Differential heating only exacerbates this problem. You might be able to use a katana as a crowbar up to a certain point, but once it takes a bend, that’s it. You’re fucked. 

A bent blade can still be pressed into service if you’re desperate, sure, so long as you don’t mind it being drastically weaker and far less effective. You might be able to bend it back, but there’s no guarantee that it won’t break, or that your opponent will give you enough time to straighten it out. 

Luckily for samurai, that wasn’t the intended use case for the katana. It was designed to be really good at one thing: slashing at unarmored targets. And at that, it excelled. 

I know I’m starting to sound like a broken record here, but no weapon that saw prolonged and repeated service on the battlefield could truly be considered bad. Well, except for maybe the Chauchat, but no one really knew how to do machineguns back then, and the French weren’t exactly in a position to be picky about it. 

The point is, you have to use a weapon in the manner in which it was intended or it’s going to struggle, and the katana is no exception. It has no magical properties that allows it to bypass that one simple fact, no matter what anime will try to tell you.