@AlexEliot
You’ll note that definition of unit is informal, and that I already said such exceptions exist. The way unit is used in the definition of cavalry that you linked is obviously not meant to be the informal usage. End of the day, in a military context “unit” does not refer to a single...
@AlexEliot
Look again at the definition you're using. It doesn't say what you think it does. "Unit" in military parlance does not refer to an individual soldier - it refers to a group or set, as you put it. That's why you hear soldiers ask each other what unit they're from - their "unit" is the...
Cavalry is a mass noun - every instance of "cavalries" in this chapter should just be cavalry. I don't know why, but this specific mistake drove me nuts.