The Icilius mentioned by Casanova, btw, is Karl Guichard, a military historian who was nicknamed Quintus Icilius by Fritz because of the following anecdote. Fritz and Guichard were having an argument about the name of some Roman general. Fritz was insisting it was Quintus Icilius, Guichard was insisting it was Quintus Caecilius. Well, the professional Greco-Roman historian was, um, right, as you might expect, and Fritz lost that argument. And then he announced that Guichard would be called Quintus Icilius henceforth, and so he was. [I've also read a less interesting variation on this anecdote, but I choose to believe this one.]

They had another argument at some point, which Fritz won. Guichard insisted that the Roman soldier carried more weight on a march than the Prussian soldier and that therefore the Romans were superior. Well, you can imagine how Fritz reacted to that. He made Guichard put on the full kit of a Prussian soldier and stand at attention for an hour. Guichard found the task so difficult that he had to take back what he'd said. He was furious with Fritz, but, really, what did he expect?