he sees his boots the first day in battle splashed with blood and urine as a soldier falls at his feet amazed – how stupid he looked! – at the sound of artillery (lines 44-47)
The only real image of war in the poem is this one. It's a bit of a flashback, as Trujillo is remembering the first time he ever fought in the military, which (we assume) happened a long time before he rose to his dictatorship. The one thought Trujillo has about the first person he killed is "how stupid he looked," which tells us a fair amount about how the man thinks about killing people – all too lightly.
He will order many, this time, to be killed for a single, beautiful word. (lines 70-72)
The last lines in the poem connect the whole piece to the history that's implied in the epigraph way back near the title. Since the poem doesn't directly talk about the massacre itself, this implication solidifies the piece's subject matter. It looks towards what's about to happen, in other words, by way of examining the situation immediately preceding it. But we know what's coming, so ending the poem before it even happens is ominous and chilling.