If the quote instead concluded with "The Marines verifiably know they do make a difference." there wouldn't be much wrong with it, other than what Nancy said below -- (one should strive for a positive difference, not just a difference).
But as it stands the quote just says the Marines no longer wonder about this, and presents it for a good thing. A surgical removal of all independent thought and/or all concepts of morality would just as easily lead to the same result. "Do not wonder about things, just trust your leaders." Pfft.
I was thinking that it could be interpreted as the Marines knowing they don't make a difference. It reminds me of this quote:
Dr. Nefarious: "To think, they called me insane, Lawrence. We'll see who's insane when my [mutant armies] have exterminated all life on this miserable planet!"
Lawrence: "That should clear things right up, sir."
Jacobi, an eminent algebraist, was fond of saying: “Invert, always Invert”
- Charlie Munger
So based on the above and the regular "rationality quotes" feature I figured why not try a thread for quotes that are not rational? Obviously we do not plain stupidity, but I think quotes that appeal on first listen would qualify. As a side benefit of analyzing specific errors this might serve as an inoculation from rhetoric.
I am using "arational" rather than irrational to highlight this distinction.
PS. Please feel free to reply with meta comments on whether this is a valuable idea.