In the phrase "gas-shells, dropping softly behind" from Wilfred Owen's poem "Dulce et Decorum Est," the literary device used is oxymoron. The juxtaposition of "gas-shells," which are typically associated with destruction and violence, with "dropping softly," suggests a contrast between the expected brutality of warfare and the gentleness implied by "softly."
This oxymoron contributes to the poem's tone by highlighting the grim reality of war. The phrase encapsulates the peaceful word choice against the horrific backdrop of gas warfare, emphasizing the dissonance between the romanticized notions of battle—often perpetuated in propaganda—and the stark, painful reality experienced by soldiers. This clash heightens the poem's overall atmosphere of irony and critique regarding the glorification of war, ultimately reinforcing Owen's message about the true horrors of combat and the falsehood of the phrase "Dulce et decorum est pro patria mori" ("It is sweet and proper to die for one's country").