Au contraire; not a mistake by you in any way. In fact I was being a bit 'liberal' in referring to Verdun, as the actual exhortation to the French garrison was "Ils ne passeront pas!" (They shall not pass!). When I say "exhortation" I do of course mean "death sentence".

Similar cries perpetuate throughout literature, films and plays, of course e.g. in "300" - surely the greatest movie about the battle of Thermopylae ever made!. And indeed in "Lord of the Rings", as you describe. Difficult therefore to pinpoint the original source, but I've always liked to think that Tolkien had The Great War in mind when he wrote the passage.

My mistake, I misquoted the source I actually meant, which is Tolkein, The Lord of the Rings, book I: The Mines of Moria. I am of course thinking of Gandalf standing against the Balrog on the Bridge of Khazad-dûm.

The Balrog reached the bridge. Gandalf stood in the middle of the span, leaning on the staff in his left hand, but in his other hand Glamdring gleamed, cold and white. His enemy halted again, facing him, and the shadow about it reached out like two vast wings. It raised the whip, and the thongs whined and cracked. Fire came from its nostrils. But Gandalf stood firm.
'You cannot pass,' he said. The Orcs stood still, and a dead silence fell. 'I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass. The dark fire will not avail you, flame of Udûn. Go back to the Shadow! You cannot pass.'
The Balrog made no answer. The fire in it seemed to die, but the darkness grew. It stepped forward slowly on the bridge, and suddenly it drew himself up to a great height, and its wings were spread from wall to wall; but still Gandalf could be seen, glimmering in the gloom; he seemed small, and altogether alone: grey and bent, like a wizened tree before the onset of a storm.
From out of the shadow a red sword leaped flaming.
Glamdring glittered white in answer.
There was a ringing clash and a stab of white fire..."

Edited by chateauferret (07 Nov 2017 6.14pm)

Indeed. Tolkein was invalided out of the Somme in 1916. Just as well.