Why were greatswords used in the Middle Ages given their downsides?

Question

The greatsword is a type of two-handed sword that's rather large and heavy compared to it's counterparts, but delivers a more powerful swing. The greatsword doesn't seem to have significant advantages to compensate for its downsides. For one, it's heavy, designed to be wielded with two hands unless the wielder is extremely strong, which eliminates the possibility of an offhand shield. Its swing time is slow, telegraphing moves and giving the opponent time to dodge or block the swing. Meanwhile, the only apparent advantage of the greatsword seems to be that it has greater range and power than a one-handed sword.

What were the reasons for the greatswords widespread use in the middle ages?

swordencyclopedia.com gives a few examples but doesn't answer my question, though, because their example of how it was used is rather shaky given my said drawbacks.

Answer 1

The "great sword" was an evolution of the longsword (a two handed sword, no matter what Dungeons and Dragons tells us).

the weapon is most commonly depicted and taught with both hands on the hilt.

It appears almost exclusively in Scotland and central Europe for a couple of centuries (after the Middle Ages!!) before dying out.

The greatsword was by no means a common weapon. Forget its effectiveness for a moment: metal has always been expensive, and quality swords were the weapon of nobility. A big sword would be even more expensive, and therefore only the wealthiest could afford such a weapon.

The wealthy also bought a little thing called "armor" (what we call "full plate" was also a Renaissance invention) that made a shield unnecessary. Indeed,

It remained in use as a weapon of war intended for wielders wearing full plate armour

On the battlefield, greatswords were the domain of shock infantry. This kind of soldier was used similarly to heavy cavalry: you run them into the enemy's weak point, breaking formations and letting the regular line of battle overrun the opposition. This specialized usage required the greatest reach and striking power available, at the cost of everything else. Notably, their main opponents - pikemen - also didn't use shields because they would be wielding loooong two handed pikes! They would be wearing exactly the same plate mail (though likely lower quality) and rely on that, as well as the range of their pike, to protect them.

This is why any martial artist would laugh at "only range." Range is the deciding factor in a fight, all other things being equal. The greatsword remained a popular dueling weapon even after its effectiveness on the battlefield was eclipsed. The linked Wikipedia article goes into longsword fencing techniques in significant depth and should clarify the potency of this deadly weapon; what you call the "greatsword" is the same but more so - it was not merely used to "swing" but also to parry, jab, half-sword, and even strike with the pommel (wielding the weapon by the blade like a hammer).

Answer 2

You can do point-defence using a greatsword, see this body-guard scenario demonstration, or this narrow alley demonstration.

The greatsword is a specialized tool/weapon for specialized tasks. Think, in modern terms, of an RPG: great if you want to achieve a task for what is was designed for; bad, if you are trying to use it in an indoors bar fight.

It is important to keep in mind, that a greatsword is not a weapon for one-vs-one fights, much like, in modern terms, a heavy machine gun isn't generally used to engage individual opponents as a sniper does.

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The mode of use for a greatsword is to keep it in constant motion, hence covering an area with a sharp blade, essentially creating a kill zone around yourself. This is very good for blocking a narrow alley, or protecting a high-value person (if the greatsword wielder is a bodyguard).

A greatsword is not suited to serve as a sidearm, as smaller swords, knives or pistols are. You simply can not wear a greatsword and retain the ability to do other things, much in the same way modern soldiers don't wear heavy machine guns as their sidearms.