Dragonlord
Fantasy Warlord
Guerilla warfare is the only tactic that can call itself a weapon. (metaphorically speaking)
But if you're going to get all technical, then I'd have to say the atomic bomb.
How do you figure guerilla warfare as a 20th century invention? Those tactics have been around a long time, but certainly in the 19th century: Geronimo's tactics immediately spring to mind, for example.