"Maybe come over here so I don't disturb anyone who's not into violence. Now, my world's magic isn't really good for war. At least, not directly. We have to use fire, pointy sticks, and our brains for that. But we're kind of alarmingly good at it, even in ancient times..."
And he gives a quick overview of ancient warfare, thrown stones and javelins and swords and bloody fists and crude armor, and starts in on a dramatic and graphic rendition of a Roman skirmish with Franks who were wholly unprepared for the invincible, unstoppable force of the Roman formation of shields-and-spears, the clever tactics of the Romans used to herd their foes like animals, pressing them against a cliff, using fire to panic them and keep moving forward until the Franks have nowhere to run, forcing them to surrender or die, the calculating, smug expression on the Roman commander's face as he counts his new slaves...