The lure of riches and political power drew soldiers to Italy continually until larger, powerful neighbors, namely France and the Holy Roman Empire, intervened - just as Machiavelli feared.
There are glimmers however, and some loose connections that can be made.
The founder of the German school of thought on the use of the longsword was Johannes Leichtenauer. He did not invent the longsword's use, but he did travel about and get information on its use. A poem of his techniques is attributed to the year 1389.
In Northern Italy, a complete fighting manual, with verse and pictures, was created by Fiore de Liberi and printed in 1410 for Nicolo D'Este, master of Modena, who incidentally had replaced a retiring John Hawkwood as leader of the Papal forces opposing Visconti expansion.
Fiore's formative years were during the years when mercenaries, such as John Hawkood and Albert Sterz, were making names for themselves in Northern Italy. These Great Captains were not foolish men, and if they sub-contracted, there is no reason to think they would not want men of skill. Masters of various weapon systems would have been in high demand. More importantly, masters would have come from all over Europe.
Fiore could very easily have encountered men who had been taught, or at least influenced by Leichtenauer and the author of MS Harely or their contemporaries. Englishmen, Germans, Bretons and more all brought to Italy soldiery, there is no reason to think they did not bring their techniques with them as well.
Fiore, for instance, is clear in that his fighting manual covers the safest and best techniques, not all of them. Fiore's idea of safe and best is not necessarily what Liechtenauer would say was best, or the author of MS Harely were they ever to meet.
Were the Manuals on Combat By Mercenaries for Mercenaries?
MS Harely's audience can only be guessed at, though it can be read to be a system that could tackle multiple opponents in what might be a skirmish setting, as portrayed by Benjamin Bradak and Brandon Helsop's 'Lessons on the English Longsword', or it could be simply a series of drills that encourage changing direction. There are few clues!
What is certain, is that the manuals were not for mercenaries nor by mercenaries. Mercenary masters may have influenced them though, especially in the case of Fiore given the atmosphere he lived in.
Mercenaries from many nationalities, in particular England and German speaking kingdoms were active in the mid to late 1300's in Italy. These mercenaries were in many cases veterans of the Hundred Years War.
Some weapon systems were too difficult to teach. The use of the longbow, for example, required too many years and almost a 'lifestyle' to use properly. Other weapon systems, namely the use of the longsword, was easier to pass on.
Fiore, one of those masters, lived in Northern Italy during the time when mercenaries were prevalent and traveled around to learn his craft. Given how many soldiers from various nationalities were active, there is no need for him to have left Northern Italy to learn the art of combat.
Mercenaries were not the intended audience of the written manuscripts by Liechtenauer, Fiore and whoever wrote MS Harely 3542. The intended audience was literate, probably noble in origin (as was the case of D'Este), and looking at Fiore and later German works, probably interested in judicial duels and self-defense- though battlefield use, especially in skirmish scenarios, cannot be entirely ruled out.
Mercenaries probably influenced Fiore's writing, the alternative would involve Fiore turning a blind eye to the sea of experience around him. All the professional warriors of his day were mercenaries, and at the time the majority were not Italian. While the mercenary masters did not write their techniques down, it would have been pointless, Fiore did and compiled them into a 'best practices' manuscript that he adapted to his audience.
This leads to the conclusion that the use of the longsword was less national in style in the case of Fiore, but rather a collection of techniques from many nationalities brought over by the great mercenary influx of 1361. These techniques stemmed specifically from German speaking soldiers and English, both of which were common in Northern Italy and would have been hard to ignore. Italian masters of arms assuredly existed before, and assuredly after this influx, but Fiore would have to willfully exclude experienced foreigners if he were to create a specifically Italian style.
Records on mercenaries, despite their long use in Northern Italy is spotty. Even the most illustrious of mercenary captains were considered outsiders. John Hawkwood was considered by the Florentines as a savior from Milanese domination. However, they ensured he stayed outside their city-walls whenever possible. The common mercenary soldiers are poorly documented. This may be because the line between mercenary and bandit, especially in 14th century Italy, was very thin- and who wants to write about how bandits swing their swords?
NOTE: the labels for this post (there weren't any) have been added by me. - B.