--- (update 13 Dec 2023) the earliest match using “war” attributed to popular author Fannie Hurst by QuoteInvestigator, 2 Aug 2021
--- (original post 4 Nov 2021) attributed to Leon Trotsky for X = "the dialectic" and "war", see Wikipedia entry
From QuoteInvestigator (QI)
The earliest match using “war” located by QI appeared in the “Cleveland Plain Dealer” of Ohio in 1941. The popular author Fannie Hurst used the expression while addressing a “Freedom Day” rally in Cleveland. Boldface added to excerpts by QI:
“We may not be interested in this war, but it is interested in us. I’m not trying to sell it to you, but no one can evade the fact that we are in the path of the storm. We dare not be disunited when liberty, the most precious jewel in our national strongbox, is at stake.
Below are additional selected citations in chronological order.
QI has found no substantive evidence that Leon Trotsky employed the saying about war. QI conjectures that the attribution to Trotsky occurred via a multistep process that began with statements about dialectics.
From Wikipedia, accessed 4 Nov 2021
This was attributed to Trotsky in an epigraph in Night Soldiers: A Novel (1988) by Alan Furst but it may actually be a revision of a statement earlier attributed to Trotsky: "You may not be interested in the dialectic, but the dialectic is interested in you." Only a very loose translation of "the dialectic" would produce "war."
...
In a later work, Just and Unjust Wars: A Moral Argument with Historical Illustrations (2000) by Michael Walzer, the author states: "War is most often a form of tyranny. It is best described by paraphrasing Trotsky's aphorism about the dialectic: 'You may not be interested in war, but war is interested in you.'"
This statement on dialectic itself seems to be a paraphrase, with the original in In Defense of Marxism Part VII : "Petty-Bourgeois Moralists and the Proletarian Party" (1942) — where Trotsky publishes a letter to Albert Goldman (5 June 1940) has been translated as "Burnham doesn't recognize dialectics but dialectics does not permit him to escape from its net."
