Look at the manuals you use. Or you will sooner or later use, if you do HEMA :-) Most manuals of the Society of Liechtenauer come from past 1400. Fiore, 1410. 1100? Highly doubtful.

And, it is a knightly weapon. That's why all these authors were called "von" like Peter von Danzig. From their angle what most people who weren't knights could afford did not matter much.

As for the battlefield, the primary issue is that without a shield or good armor you get shot to pieces with arrows before you even join the action. See Falkirk (which was a "schiltrom" but with long spears / pikes, probably two handed, so without a shield...) Compared to that, fighting some guy who has a shield and a side-sword is far less scary.

Notice something: it is a knightly weapon, yet unwieldy on a horse. How often do knights fight on foot?

Well, the answer is, English knights surprisingly often, Crécy, Poitiers, Agincourt... the longsword saw a lot of usage there.

Then English mercenaries went to Italy and that is how we got Fiore. They brought their "secret weapon": armored knights fighting dismounted.

But by the time Fiore wrote his magnum opus the longsword was becoming outdated for the battlefield. Its best era was really the Hundred Years War, Crécy and so on. The reason? Armor was getting too good. They went on to bigger can openers, such as Swiss pikes or bludgeoning weapons.

Plate armor was getting widespread amongst knights - and that is something already too hard to deal with a longsword. Hardly any chance to cut, and you really have to aim your thrusts accurately and with a strong force. Hence Fiore invented halfswording, which is really a hack, not a solution.

German mercenaries fighting in the Hundred Years wars and Italian wars imported the longsword. Hence we got Liechtenauer. But unlike Fiore, Liechtenauer was not writing for the battlefield commander. He was about to convey some kind of a "secret art" to his students which is more of a self defense or duelling kind of stuff. Like some awesomely secret kung fu master, his primary goal was not about winning full on battles. Longswords in Germany became a tool for judicial duels and sport. The reason you get so many German manuals which made it fairly easy to resurrect this sport is that it was, well, a sport (although a bloody one in duelling), not primarily a battlefield tactic in Germany.

Good summary

I am not sure it answers your question fully. Again: when plate became widespread the longsword was proven too weak. Thus, probably most users did not have one. They typical user was a dismounted English knight in the 100 Years War. Whose longbowmen were far better than French archers or Genovee crossbowmen and thus the knights did not have to fear arrows much. The rest was more or less fencing. Yet, they were pretty decently armored, even though not in plate, at least not in the beginning - this war saw a continuous evolution of armor. Visby, 1367 was largely this type of coat of plates. Later on during the war, it evolved towards the plate.

There are really two questions here:

1. Were Longswords used before the development of full plate armour?

The answer to this is yes. The earliest known longsword at the Glasgow museum is dated between 1250 and 1300. Some limited plate defenses are known from this early period (there are depictions of plates under surcoats, and some written accounts referring to something like a coat of plates). However most sources wouldn't call 13th century armour, which was maille with plate reinforcement, 'full plate armour.' Moreover, of the swords in the Glasgow museum, a number of them are dated to around 1300, which is still early in the development of plate armour; there are funeral brasses of knights without visible plate limb defenses as late as 1331 in England (see the Brass of Sir Robert de Bures). The first plate armours that most people would describe as 'full' are from the later 14th century, as in the altar piece of Pistoia Cathedral, from around 1376. However, the longsword -did- become more common after 1400, when full plate armour becomes common throughout Western Europe.

2. Did people use longswords to fight when unarmoured?

The answer to this is also yes. German fencing manuals make a distinction between Bloszfechten and Harnischfechten - the latter is fighting in armour, specifically. The techniques are distinct, and they are illustrated separately in fencing manuals. The 'normal' techniques include a lot of cuts that would greviously wound an unarmoured man, but bounce off of plate armour. The armoured fighting techniques, on the other hand, rely heavily on 'half swording' - gripping the sword halfway up the blade - to better control the point and finds gaps in armour. So the German masters of fencing certainly taught techniques for fighting unarmoured.

I hope that answers your question. I do not know of longswords from before the 13th century, but I'm mostly a Late Medieval/Early Modern guy.

As a final note, however old the form of the weapon you are using is, the techniques that you are learning are only documentable in the late middle ages - we have no manuals in this school from before the end of the 14th century, and the earliest surviving fencing manual that I am aware of is from 1300 (i.33, which deals with sword and buckler fighting).

Sources: The Real Fighting Stuff, Tobias Capwell - dating on Longswords at the Glasgow Museum

Arms and Armour of the Medieval Knight, Edge and Paddock - Overview of armour development and references for effigies and other depictions.

I referred to this Transcription of Talhofer for terminology - it also illustrates armoured versus unarmoured techniques.

As an illustration of armoured fighting, I like this one YouTube video from the Musee de Moyen Age in Paris: Le combat en armure au XVe siècle

Of course they were often used without the benefit of full plate armour. Full plate suits only started appearing in the very late 14th and early 15th century and they reached their zenith in the 1500s. Longswords were around for a very long time before that.

There is no real intrinsical connection between a suit of armour and a longsword. They are two separate things. A longsword, in the nail sense, is just a fairly large sword that can be wielded with two hands. That's it, really. Not having a full suit of armour, any armour!, be it mail or mail with plate augmentations or an actual, full suit of plate, does not reduce the effectiveness of the blade, nor does it take anything away from its characteristics.

Not quite sure why you seem to think longswords and plate suits are linked like that. If anything, a sword is not a particularly great weapon to take on somebody in a full plate suit. This is why, as armour becomes stronger and more prevelant through the high and late middle ages, we see more blunt weapons appearing such as maces and warhammers.

To add to /u/mhollender 's answer, I'll try to answer "Why would I go into battle wearing something like mail"

Armor protects in three ways.

The most obvious is the tensile strength of the material resisting penetration and spreading the force of impact over a wider area of body.

Another method is by deflecting the blow. This is what "plate of proof" was most renowned for. If something strikes it at an angle, only a portion of the force (cosine of the incident angle) can be delivered to the armor. As the armor itself will bend, and the wearer also move away from the blow, this reduces it even further.

Finally, armor stops weapons and projectiles simply by adding mass that needs to be deflected before it can impact the body. The most famous modern example would be the depleted uranium on M1A2 tanks. DU is a very soft metal - but it's incredibly dense.

Mail - quite common before plate - doesn't perform as well as plate, or even many of its contemporaries at the first two (I've read claims that even boiled leather is better), but it excels at the third so long as you're not being hit where the armor is actually resting (your shoulders).

With mail, when a weapon hits it, it doesn't just have to move the links it's striking - it has to move the links to the left, right, above and below the point of impact, and this can be quite a lot of weight in comparison to a 2-3 pound blade. And it still has the gambeson to deal with before actually reaching skin.

This sub looks down on anecdotes, unfortunately I don't know if there is any proper way to answer this. If you do end up actually sparring, watch how often someone actually catches you in the shoulder, versus just about anywhere else. If your shoulders are magnets for the weapons of your enemies, then you are likely right to be afraid and wearing mail into battle may not be for you.