Can the term modernist, when describing literary work, be in fact used to reference poetry that was, but not is, modern?
Beg pardon, but which society are you talking about? Your premise would not be true about "all" or even most British poetry in World War One. In fact, during the anniversary the other day people were saying how the great war poets like Owen were more anti-war than the average. Germans, French, Russians, Austrians and Americans, I wouldn't know. Without speaking Italian, however, I am aware of the Futurism of Marinetti & Co., which might be called a variant of Modernism. Not exactly the poetry of the war, and more about art than poetry, but one response to the cataclysm.
Anyway, a more English expression would be that the poetry was "pro-war" or "militaristic", or that it "glorified war" or "romanticised war". It's harder to do as an adjective.