I see a lot of soldiers with arming swords and no bucklers in artwork, but treatises seem to focus on longsword and sword and buckler, with only some 16th century treatises showing sidesword in one hand. So my question is was it common for soldiers to use arming swords without bucklers? More or less common than with a buckler? And if it was used, how come there doesn't seem to be any treatises dedicated to it?

Hi Adam. AFAIK in some of the 15th century manuscripts that teach sword and buckler. Some times a sword is used, and other times a messer is used. So it could be that these two weapons were seen as fairly interchangeable. If this was the casse the there wouldn't be any need to dedicate a section of a treatise to arming sword, were messer was already covered.

Fiore has a section of his treatise dedicated to the sword in one hand.

Well Fiore doesn't really teach single handed/arming sword. He teaches the use of longsword in one hand. Though I'm sure Fiore's techniques would need little, if any, modificationfor use with an arming sword.

And of course people could perfectly well go hacking away with their swords without any prior systematic training. The fact that medieval martial arts existed doesn't mean that everybody knew them (although of course they were probably far more widespread than they are today).