I have been reading Ross King's Brunelleschi's Dome: The Story of the Great Cathedral in Florence. Although the book is well documented, it has an astonishing passage for which the author provides no source. The context is a war between Florence and Milan. At that time, Milan was ruled by the Duke Filippo Maria Visconti; he is the person mentioned in the first sentence of what follows.
The following year, as plague raged through Tuscany, his forces defeated the Florentines at Zagonara, in Romagna. There were only three casualties, all Florentine soldiers who fell from their horses and drowned on the battlefield in their heavy plate armour (it had rained heavily in Zagonara the night before). This lack of bloodshed shows that warfare in the Middle Ages and Renaissance, contrary to popular misconceptions, could be reasonably civilized. Most battles resembled chess matches in which opposing commanders sought to outmanoeuvre each other, the loser being the one who conceded that his position was technically vulnerable. These engagements were fought by mercenaries who setted the terms of warfare, rather like sportsmen deciding the rules of a game.
Is this description accurate? If it is, where can I learn more about this way of waging wars?