Après quelques recherches, j'ai trouvé la phrase qui a inspiré cette falsification (Butler, 1893, 85):
The nation that will insist upon drawing a broad line of demarcation between the fighting man and the thinking; man is liable to find its fighting done by fools and its thinking; by cowards.
Elle fait partie d'une série de citations érronées de Thucydide relevées par Morley (2013, ici 32):
The usual response by those opposed to “Neo-Realist” or “Neoconservative” readings of Thucydides has been to argue for a more nuanced and detailed reading of the text in place of a few decontextualized and misinterpreted maxims, but such a response—however historically and philologically credible within academic circles—is entirely inadequate in the face of the rhetorical power of “bumper sticker philosophy” in the sphere of public discourse.
Butler, William Francis. 1889. Charles George Gordon. Vol. vi, 255 p.English men of action. London and New York: Macmillan and co. https://doi.li/qg4jog2e
Morley, Neville. 2013. "Thucydides Quote Unquote." Arion: A Journal of Humanities and the Classics 20 (3): 9-36. https://doi.org/10.2307/arion.20.3.0009.