"You don't know what you're talking about! Dodging an attack is nothing more than a fighting instinct. It's a natural reaction developed from years of hand-to-hand combat, but it changes nothing: No matter what you do your sword can not cut me!"
The less actively a combatant pursues his own defense in a fight, the less likely heneedsto.
The corollary: Any attack that forces someone who normally ignores self-defense to dodge the attack is an attack that can kill him in one hit.
Frequently seen in Implacable superheroes/villains and Big Bads trying to make an impression. The Logical Extreme is the No Sell, when the recipient of the attack makes no effort whatsoever to avoid or block it, and doesn't react at all when it hits.
Although there are some real-life examples, attempting to use this in Real Life is more likely to cause quick and stupid deaths. It just doesn't work if you do it on purpose.
The sad (terrifying) fact is that this is an instinctive response called The Peltzman effect, where the person will take more risks according to how safe he is in some kind of internal (insane) sense of balance. But since humans tend normally to overestimate their benefits... well. Worse is that it's a constant effect that people do without realizing day after day. Not so different from pain tolerance; expected pain "hurts" less than surprise pain.
Compare Punch! Punch! Punch! Uh Oh.... See also Good Thing You Can Heal (when fighters with a Healing Factor don't bother defending because "meh, I can grow that spleen back") and Nonchalant Dodge (a fighter expends the minimum effort to avoid an attack, but doesn't seem remotely fazed by it). A Not So Invincible After All moment usually occurs when said character bumps into the thing that poses a genuine threat.
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Very common in the Dragon Ball franchise, whether between one of the ridiculously powerful main characters and an ordinary human, or by a villain to show how far above the hero they are. As early as the second episode of Dragon Ball Z, Raditz makes light of Piccolo's power, stands there and allows himself to be blasted, and when the smoke clears...
Raditz: Goodness! You've managed to singe some of my leg hairs.
Maybe that wouldn't happen if he wore pants!
Played with by Cell who (despite Vegeta's warnings) stands still to take a Final Flash. As he is about to be struck, he cries out in surprise, and moves aside, but his arm is blown off. Vegeta, seeing the damage he's done, is very pleased with himself - until Cell reveals that it was all a ruse, regenerates the arm, and proceeds to beat the snot out of Vegeta.
Kenpachi Zaraki from Bleach. He's perfectly content to let his opponents attack him, and actually encourages them to go for the eye or the throat. The first time Ichigo tried to hit Kenpachi with his sword, Ken was unharmed, and Ichigo's hands got hurt from the impact.
Also, Nnoitra Jiruga fits it pretty well, especially the corollary during his fight with Kenpachi. He doesn't actually dodge any of Kenpachi's attacks until Kenpachi aims for the head, giving Kenpachi the proof he needs that Nnoitra's hierroisn't actually invincible.
Aizenaverts this for a while, using his illusions to avoid attacks he could probably easily tank anyway. But when he reveals his Healing Factor, he adheres fully to this trope.
During Fate and Signum's first battle, Signum calmly put a barrier around her and just stood there with her eyes closed as Fate launched Photon Lancer. The energy missiles just bounced off of her. Later in the battle, when Fate charged at Signum with her axe, Signum actively dodged it. In the next episode, it's revealed that Fate had managed to wound her then.
Averted with Reinforce (Eins), who isn't averse to using external barriers to stop her opponents' attacks. It makes things all the more Awesome when Nanoha pierces her barrier, fires an Excelion Buster point-blank at her... and she isn't hurt at all.
In StrikerS, Fate develops a "defensive costume" that trades all protection for a massive speed boost. The Numbers' best fighter immediately recognizes that a single hit that connects will kill Fate. What she fails to realize is that in a high-speed battle, you CAN'T land a hit on Fate.
Yuuno was good at two things, barriers and teleportation. When he's seen fighting Vita in the second episode of As he nearly completely ignores her, throws up a defensive barrier, and keeps working on teleporting the entire gang out of the Wolkenritter's barrier. This ALL WITHOUT THE USE OF A MAGICAL DEVICE. Vita had demolished Nanoha and would have won if the others hadn't interfered just the episode before.
Alucard from Hellsing, not because he can't be damaged (rather the opposite, really) but because he heals so fast and is practically unkillable. He likely also doesn't because it's as scary as hell for those attacking him with absolutely no clear actual permanent effect. When he begins employing anything approaching tactics, you can tell the fight is very serious.
Conditional immortality. Effectively, Alucard must be killed once for every life he's consumed. So his "regeneration" is more like transferring the wounds onto a substitute body. When unleashing his full power, he actually divides into an entire ARMY of soldiers; everyone he's eaten. Naturally, since he's hundreds or thousands of years old, that's A LOT OF DYING.
Hidan in Naruto is entirely immortal, and thus has almost no strategy or moves beyond "get opponent's blood for voodoo technique". Both his partner Kakuzu and eventual opponent Shikamaru tell him how stupid that is. They are entirely right. Again, Conditional Immortality like with Alucard. Word of God says that, if he doesn't perform his ritual sacrifices, his immortality will eventually run out. No word on how long that TAKES, though.
The series is otherwise an aversion, as it's shown that even with heavy use of Charles Atlas Superpower there's a limit to the raw physical ability any person can normally achieve (as detailed in this helpful essay). Thus, even insanely powerful ninja have to avoid simple things like kunai and explosives and the strongest techniques are the ones that make being hit nearly impossible.
Hidan DOES have to dodge or avoid certain things though, since his body parts can be severed, and he shows no ability to reattach them. After getting his head cut off, Kakuzu has to sew it back on. Since Kakuzu is capable of ripping out peoples' hearts and absorbing them with that sewing... it's debatable if Hidan would've been able to pull himself back together on his own. Though Hidan's mid-range attack methods suggest not.
Some ninja, like Orochimaru, still play the trope straight though. In Orochimaru's case, he can simply regenerate after almost anything including a broken neck. This also bites him in the ass, as he makes no effort to dodge a blade that he thought couldn't harm him, only for it to turn out to be a spiritual blade that sealed him in an endless illusion.
Orochimaru also plays it entirely straight when he fights Naruto in the 4-Tailed Transformation. He makes very little effort to actually dodge attacks, though he does use a technique which allows him to create a "new" body in order to avoid certain attacks' effects after they've connected. It isn't until Kyuubi!Naruto uses the 4 TailsMenacing Ball that he puts forth a serious effort to defend himself, commenting on how it could kill even him in one shot. Said "serious effort" being to summon the Triple Rashomon Gate in the blast's path; a pair of his Elite Mooks had previously described a single Rashomon Gate as the "ultimate defense". All three gates were were destroyed, but the blast was slowed enough for Orochimaru to avoid destruction.
Suigetsu partially averts this though as despite being able to avoid damage by turning into water, he still prefers to avoid attacks in a more conventional fashion (except in that one filler episode, but that was against someone he really didn't consider a threat and to scare him by showing the power off). Granted, this might simply be because he can't attack while reforming or a limit on how much he can regenerate before losing consciousness/dying.
Subverted with both Gaara and later Sasuke who tried to rely on their "ultimate defense" (the Sand Shield and Susanoo) technique to mean they don't need to do anything else to defend themselves, but eventually have both of them defeated.
Played straight with a twist in Blade of the Immortal, as the main character, the titular immortal swordsman Manji, is bothered by the fact that his reliance on his regeneration induced immortality means that, consequently, his sword skills are slipping.
Subverted in Tengen Toppa Gurren Lagann: Simon puts up a barrier of Spiral power that effortlessly deflects a barrage of Anti-Spiral attacks. He mocks the Anti-Spirals when they immediately try another barrage - but this one penetrates the barrier and causes serious damage.
This is an example of of the Anti-Spirals rewriting the laws of the universe, basically, to make "the impossible possible."
Justified in Rurouni Kenshin, where Kenshin's best Hiten Mitsurugi Ryu moves have two parts, greatly reducing blind spots and successful counterattacks.
In Slayers, to demoralize Lina into using her Dangerous Forbidden Technique, Hellmaster tells the Slayers that they can attack him all they want and he won't counter-attack or even attempt to defend himself, and then spends a while laughing off everything they throw at him (even more so in the novel version, where all of the Slayers actively participate in the fight). The first thing that forces him to react is two simultaneous Dragon Slaves from Lina and Sylphiel, which "might have hurt a bit if I hadn't dodged it". The only thing that he really considers worth defending against is the Giga Slave - at which point he discovers that ultimate destructive power is not a toy.
Averted in Black Butler, where Sebastian regularly dodges when shot at despite being Immune to Bullets. This is suggested to be because getting shot ruins his clothes, and as a Battle Butler he must look impeccable.
Father from Fullmetal Alchemist can do pretty much the strongest alchemy in the entire series without moving at all. He spends several chapters just standing there, arms crossed, blocking everything the good guys can throw at him. And when he finally raises his arm to block, that's when they know he's finally losing it.
Accelerator from A Certain Magical Index has control over vectors, so anything that makes contact with him will bounce off, even if it hits him without him being aware of it. His arrogance bites him in the ass in his fights against Touma and Amata Kihara.
As a testament to Accelerator's Character Development, much later in the series, he faces Umidori Kuroyoru. Kuroyoru launches an attack that can hurt him, expecting Accelerator to take the attack and get himself killed. Accelerator dodges, causing her to have a Villainous Breakdown as she didn't see that coming.
In Fist of the North Star, Souther's Nanto Ho'oh Ken has no defensive stance. In the initial fight against Kenshiro, he manages to No Sell every technique Kenshiro uses (due to having every pressure point on his body reversed due to his heart being on the opposite side.
Juvia from Fairy Tail has a body of living water that normally feels no pain and can effortlessly re-from when splattered. When Meldy throws a Storm of Blades at her, Juvia confidently stands still, but they turn out to be magical swords that can hurt her, and she gets badly injured.
Totally averted in Chapter 280 of Kenichi: The Mightiest Disciple. The Karate club is taught Muy Thai by one of the members of Yomi, but he only taught them how to attack. As such they could not defend against Kenichi at all when he counter attacked.
Often used in One Piece: Luffy is made of Rubber and thereby immune to bullets and blunt force trauma (unless supplemented by haki), Logia users are elementals and immune to everything except sea stone, the sea itself, something that defeats their element (e.g. using water on a fire logia) or haki.
Both of these come up as plot points later as Crocodile (a sand Logia) and Enel (self appointed god of Skypeia and lightning Logia user) Don't bother to dodge luffy's punches they get smacked in the face as Luffy has coated his hands in water or his own blood (crocodile) or is made of rubber and is insulated from electricity (Enel). From this point on both fights are over pretty quickly as neither is as used to conventional hand to hand combat as Luffy. Likewise Luffy makes little to no effort to block punches so when Sentoumaru shows up and uses hand to hand combat, reinforced by armament haki on him Luffy is freaked out that he's being hurt by these attacks
Superman in, well, just about anything he shows up in. This was lampshaded in the DCAU by Batman, who realized that Superman is so powerful he probably doesn't need to fight strategically most of the time.
In The DCU proper, kryptonite-tipped bullets have been used several times against Superman for that very reason. While he could very easily dodge a bullet, he's simply got no reason to and will therefore stand there and let you shoot him. Get a bullet that hurts him, and this backfires against Big Blue rather badly. But then, Superman being the kind of person he is, he's likely tanking the bullets just to protect any innocent bystanders nearby.
Lampshaded by Jerry Seinfeld in a American Express TV commercial with an animated Superman. Instead of catching a VCR thrown at him, Superman just puts his hands on his hips and thrusts out his chest. Jerry asks him why didn't he just catch it (to prevent it from breaking) and Superman replies that he doesn't catch, he does the chest thrust.
Lampshaded in the recent Superman Returns movie, where Supes wins a "fight" with some bank robbers merely by showing up.
Clark in Lois and Clark was supremely bad about this, in a show where the number of laser guns and other Mad Scientist inventions outnumbered the Kryptonite chunks, Superman would just stare at the bad guy's gun pointed at him instead of dodging it or using his heat vision to melt it away.
Adam Destine (ClanDestine, Marvel) can't be harmed, period. As he has lived several centuries with this condition, he is tends not to react to any attack. His usual tactic is to stand still while his opponent gets tired or leaves.
Kimura is indestructible, so she doesn't even flinch when a gun is pointed that her head, or when faced with X-23's adamantium claws. Her arrogance bites her in the ass when she faces Emma Frost.
Paul in With Strings Attached. Given that the super-strong Paul has absolutely no interest in hitting anyone (unless one of the others is threatened) and no combat training whatsoever, he has no moves at all in case something penetrates his invulnerability, as with the wraiths or the Hunter's BFS.
Robin Hood in Mel Brooks' Robin Hood: Men in Tights does something similar, having a nonchalant conversation with his servant while fighting the guards.
His blind servant, arriving in the middle of the fray to deliver Robin a beer, who drinks it in one hand while fighting mooks off with the other.
"Sounds like we're winning, sir."
The Cobra gunship from G.I. Joe: The Rise of Cobra lets anti-aircraft 50 cal bounce off its armored shell, but actively engages incoming missiles.
In Ip Man 2, the Twister asks Master Hung's students to hit him as hard as they can and laughs off their blows as wimpy. When he actually throws down with Master Hung and Ip, though, he properly blocks and evades their attacks.
In The Matrix, Neo becomes so masterful over Agent Smith that he can fight him off with only one hand. In the sequel, an Agent almost lands a blow, which Neo blocks with his other hand before very casually saying, "Hmm. Upgrades."
Sebastian Shaw in X-Men: First Class. When Magneto sends a storm of metal at him, he doesn't even seem to notice it as all the metal harmlessly bounces off him.
Which makes sense since his mutant power (in the comics at least) explicitly is to absorb kinetic energy, so simply hitting him tends to leave him both unhurt and stronger than before. He's still vulnerable to less straightforward attacks (even he still needs to breathe, for one) and knows it, though.
Invoked in the Dale Brown novel Wings of Fire, where Hal tells a Night Stalker new to the Tin Man Powered Armor to stop being obsessed with taking cover like a conventional foot mobile and focus on getting the job done while letting the suit's protection shrug off small-arms fire.
Beowulf has the titular hero starting out battling a monster naked—but by the time the dragon comes around, he's an old man and prudently equips himself with chain mail and a shield.
In The Dresden Files, Ivy the Archive is a Great Big Library of Everything in human form. She knows everything that has ever been written down. In the Dresdenverse, knowledge equals power, and she has a lot of it. In a spectacular display of power, she manages to take on seven very Elite Mooks, blocking or redirecting every attack with seemingly minimal effort. But when the leader of the Elite Mooks throws a spell at her, Ivy devotes most of her attention, as well as one of her hands, on defending against her attack.
The Big Bad of The Alloy of Law doesn't bother to avoid punches, bullets, or dynamite blasts. He even blows his head apart with a shotgun, just to make a point.
Glory in Buffy the Vampire Slayer rarely had to defend herself against anything, even when Uber-Willow was slinging major mojo her way.
In Band of Brothers, the more time soldiers spend trying desperately not to die, the more likely they are to get killed, while those who simply focus on completing their mission tend to do rather better. As in the case of Lt., later Captain Speirs, who runs through occupied Foy to link up with another company assaulting Foy from the other direction, then runs back through in order to lead his own men into the now-coordinated attack.
This is a Borg policy in Star Trek, as they will ignore anything they do not perceive as a threat. The very first drone to appear on the Enterprise makes a point of this, as it ignores not only Picard's attempts to communicate, but a low-level phaser blast.
Q: "Understand you"? You're nothing to him.
Since individual drones are just about as important to the Borg as individual cells are to humans, this does make some sense. Much less in the later incarnations, where the Borg are starting to resemble zombies led by a quite individualistic queen. Especially since Starfleet members have on multiple occasions taken advantage of the drones ignoring them to seriously sabotage their efforts.
Of course, since the Borg's hat is adapting to what's hurt them, one can take this as the Borg adapting to Starfleet exploiting the drones.
Except that, given that the Borg have been around for at least a millennium, you'd think that they would have dealt with internal sabotage before. How much does proper internal security actually cost them? At this point, it's difficult to explain their behaviour as anything other than pathological arrogance.
In the first season of Heroes, the show's supervillain Sylar would use his telekinesis as a passive forcefield to blunt the impact of surprise bullet hits, and could even stop bullets in mid-air Neo-style if given enough time to actually react. After he absorbed a Healing Factor in the later seasons, though, he just stopped bothering and let himself get riddled with holes, even though doing so clearly inconvenienced him much more than simply stopping the bullets with telekinesis.
It kind of makes sense, though. If he uses telekinesis, they'll keep shooting, hoping he slips up once. If he doesn't try to stop the bullets, then they're more likely to stop shooting, since there's clearly no point.
In the Haven episode "Fear and Loathing", Ian Haskell steals Nathan Wuornos' Feel No Pain ability, and while it makes him tougher, he assumes he's invincible. When the heroes shoot at him, he laughs off his injuries until the damage and blood loss makes him collapse and start dying.
Averted in go (a.k.a. igo, weiqi, baduk) - if the stronger player can forget defense, the handicap is too small - and according to the game etiquette, they should play weaker in order to avoid crushing their opponent. Especially if the handicap was purposefully omitted or too small. The stronger one is supposed to "gently" show where a mistake was done, and not take too great advantage of obvious blunders. It's supposed to be a sign of a good player that they can defeat a weak player of any level by a small margin.
The fluff of Vampire: The Masquerade mentions that Ventrue with the Fortitude discipline often like to use their supernatural toughness as an intimidation tactic, standing still and letting muggles throw away bullets, knives and other attacks at them (which they shrug off) as a way of psyching them out.
Pokémon Ruby and Sapphire has Shedinja. Shedinja has an ability called Wonder Guard, which makes it impervious to all attacks that its type combination isn't weak to. It also has only 1 HP, so any attack that isn't stopped by Wonder Guard, including any environmental effects or status effects, will knock it out.
Shining Force 2 has Kiwi, who starts off this way. He will take 1 damage from any attack (except magic and exceptionally strong enemies), but he only starts with 8 HP, and doesn't get much better. The main problem with his is the fact that the farther in the game you get, the more magic the enemies cast on him and his term of "exceptionally strong" gets looser and looser. He usually finished off the game with approx. equal defense as everyone else, but only half (if not less) the HP.
Berserker from Fate/stay night is big on this trope. Attacks beneath 'A'-rank cannot hurt him at all and he has 11 extra lives on top of that; he must be killed 12 times to keep him down. Thus, any attack Berserker will even deign to defend against will be one capable of mortally wounding him: It therefore becomes noticeable when he is actually forced to go on the defensive, especially in the anime when he fights Archer and the up-to-that-point unnoticeable Servant manages to force him to do just that with his first attack.
Although, Archer's involvement up to that point in the game route that the series of mainly based on is due to Saber having injured him.
Dante from Devil May Cry, at least in cutscenes, he will let enemies impale him with pointy objects, crush him underneath them etc. because of his Healing Factor, usually leaving a wisecrack or two. If he starts blocking or evading properly then you know the fight's serious.
The actual gameplay also demonstrates the corollary very well, especially on the harder difficulty modes: If you aren't jumping around like a frog on meth, you will be torn to shreds by most of the enemies you face.
In Mass Effect 2, when Harbinger takes control of a Collector, he makes absolutely no effort to hide or avoid taking damage. The Collector he takes over stands out in the open behind intensely powerful barrier and armor protection and keeps pounding Shepard's squad, ignoring gunfire, tech attacks, and biotics until his armor and shields have been completely depleted. Then he takes control of the next Collector in line. He even lampshades how little he cares that you're blasting away at him, calling your attacks "insults." At the end of the Suicide Mission, he releases control of the Collector General as the station explodes around it, implying that such a thing might have actually hurt Harbinger himself.
The death of a host body was shown to weaken or distract Sovereign in Mass Effect 1. One can assume Harbinger left prior to the deaths, or that this is phenomenon is why Harbinger has a brief lag between possessions.
If you watch the cutscenes carefully, you'll note that Harbinger took direct control of the leader of the Collectors before the first time you fight one of his puppets, and relinquishes that control just before the leader is killed. Harbinger was lying to you every time he said "assuming direct control;" he was actually forcing the one Collector he'd taken control of to control those fighting you, giving him indirect control. Because Harbinger knew what you did to Sovereign, and was Dangerously Genre Savvy throughout the second game.
Played with is Iji where late-game boss Asha, in his rematch doesn't bother to dodge shotgun or autogun rounds, believing them to be beneath him. He doesn't count of Iji surviving as long as she does, constantly chipping down his health until his pride gets him killed.
Probably the most well known real life example of this trope would be during the 'Rumble In The Jungle', the fight between George Foreman and Mohammed Ali in 1974. Foreman had knocked out almost every opponent within three rounds, which Ali capitalized on by taunting him and giving him the opportunity to take easy but ineffective body shots. Eventually, Foreman ran out of steam and Ali finished him. Ali had beaten one of the hardest punchers of all time by letting Foreman punch him until he could punch no more.
When full plate armour came into vogue it was largely proof against most of the infantry weapons of the day. Wearers generally had to cripple their opponents by crushing joints with warhammers or basically wrestle them to the ground and stab through the weak points with a broad bladed dagger or half-sword technique. Trying to kill a knight by swinging a sword at him was totally futile; he could ignore all your attacks and settle down to killing you in the most appropriate fashion. Note that this includes early firearms.
To reiterate, swinging your "standard" sword wouldn't faze a knight in armor, so you had to have the martial prowess and strength to wield weapons and techniques capable of overcoming his armor or you had to have the strength and training to overpower him and kill him in a grapple....all of these are skills that a knight has in spades.
Contrast its main predecessor, chainmail. This was heavy and bulky and inconvenient to wear, especially when backed with enough padding to absorb the impacts of weapon blows. Powerful piercing attacks from swords, lances or arrows could penetrate mail to the detriment of its wearer.
To elaborate, slashing weapons (curved blades, in other words) were almost completely ineffective against chain mail. Up to and generally including the katana. One can actually track the evolution of weapons and armor side by side like this, as cloth or leather armor folded pretty quickly against a slashing weapon. Straighter weapons become more popular as sturdier armor was developed.
A lot of old suits of armor made knights have that funny pointed pot-bellied look. Unless a stab is perpendicular to the slope of the breastplate, it would tend to deflect off to the side instead of puncturing, and naturally a conic form makes determining the angle to stab much harder in the heat of combat. Naturally, you rarely if ever see it in mass media, due to Rule of Glamorous.
The modern counterpart of the armoured knight is of course the battle tank. Early tanks did not have the armor to resist much more than machine gun fire, but that alone was sufficient enough to help break the deadlock of trench warfare in World War I. Though far from invincible - one good, armor-penetrating hit will generally knock a tank out - when used properly in shock tactics and supported by infantry, the sight of ineffectual, sporadic weapon fire glancing harmlessly off a tank as it rips through enemy defenses is a common real-life Crowning Moment of Awesome. Just don't go thinking you're completely invincible. One well-placed HEAT round can still ruin your day. Shoot first!
In the same vein as the tank, but going back even further, ironclad warships brought the same defensive powers to naval warfare, with cannonballs simply bouncing off their armour plates. Since they were armed with those same weapons, the first few duels between these ships were all draws. For a brief time, when two ironclads fought, neither ship could hope to harm one another. There was in fact a short period around that time when the idea that due to the ineffectiveness of cannons ramming would come back into fashion as a decisive naval tactic! Of course, that was when black powder cannons (frequently still muzzle-loaded) firing actual cannonballs were still en vogue — and naval artillery refused to remain stuck there long enough for the prediction to actually come true.
This is at least partly the point of the Bando Monk system of self defense (Burma?) where their goal if attacked is to dodge and parry until their attacker gives up in frustration. Then they wish him or her a nice day and go about their business. Yes, it's active defense, but the intention is to frustrate the attacker into submission.
People will often start off following all safety protocol and then after time, pay less and less heed to it. This comes from people relaxing because the dangers that the safety protocol prevents doesn't happen often. Then the disaster happens and people get hurt.
This is why "drills" are common in most industries. By running workers over and over again through the proper procedures they become instinctive actions, and the workers will (in theory) keep themselves safe without ever having to consciously think about the safety protocols.
Can also be somewhat true in real sparring and fighting situations by attacking overwhelmingly enough that the opponent goes entirely on the defensive. You don't have to defend yourself if the opponent is too busy ducking and dodging to fight back. Better hope your opponent is weak in defense, though, since attacking takes far more effort. In an unarmed striking fight, if you don't win before you tire, you're at his mercy.
This counter-strategy is the entire point of the rope-a-dope defense (and similar ones) where you let someone tire themselves out from attacking. And then while they're exhausted, you unload on them. The problem is that if your opponent knows what he's doing, and is fighting aggressively enough, you won't be able defend against all of his blows. Attacking may be more exhausting than defending, but a well performed attack will usually still break through via proper use of feigns, sheer speed, and the fact that once you take just one good hit, your reaction time goes to hell, making it harder to keep on defending. This is why defense in a close combat situation is usually a temporary measure used to launch a counterattack from rather than anything you want to keep on doing for an extended period.
One of the core principles behind the Israeli martial art of Krav Maga (besides the fact that it assumes a pragmatic Real Life approach of "no quarter" and going for vulnerable areas like eyes or groin that are normally a no-no for other martial arts) is to counterattack at the same time as blocking/parrying an enemy's attack. Most enemies will not expect someone to strike at the exact same moment their blow lands or is parried. This requires much practice and good timing, as your force will be split between defense and attack.
Completely averted in sport fencing, where an attack must be stopped or avoided before a counterattack is considered valid. Were the swords sharp, failing to defend before counterattacking would result in punctures of various vital organs, especially the lungs. Epee, which is based on a duel to first blood with less lethal weapons, is an exception.
Played with by some Medieval-and-Rennaisance-era Western martial arts, such as the Liechtenauer tradition of German longsword fight, which advocate combining attack and defense in the same movements. The ultimate goal is to defend yourself in such a way that your defense also attacks the opponent, and to launch attacks that aren't easily countered. Since one good blow generally ended a fight in swordplay, the best defense was one that killed the opponent and prevented him from attacking again. Swordsmen got very good at this - the sign of an experienced swordsman was an explosive counterattack that immediately caused the opponent to be able to do nothing but defend himself, and keep being forced to defend himself over and over until he failed to do so.