I and others have quoted Aragorn’s speech (which wasn’t in the book) in Return of the King. Not bad, and appropriate in context. But perhaps another speech by another king, this time one straight from the book, might give the neck-choppers pause if quoted by someone with authority:

“Arise, arise Riders of Theoden!
Fell deeds awake: fire and slaughter!
spear shall be shaken, shield be splintered,
a sword-day, a red day, ere the sun rises!
Ride now, ride now! Ride to Gondor!”

Not much ambiguity in that one. I have also heard suggested that the USA change its anthem to the Battle Hymn; which would be an equally powerful message. Perhaps, also, the jihadis should be reminded of another little speech, this time a real one, from Oppenheimer:

“If the radiance of a thousand suns were to burst at once into the sky that would be like the splendor of the Mighty One… I am become Death, the Shatterer of Worlds.”