The "shift" that you speak of in "Dulce Et Decorum Est" is in line nine when the speaker exclaims, "Gas! Gas! Quick, boys!--" Before this point the poem is simply a description (although a morose one) of marching in formation to some undisclosed location during World War I. However, as soon as Owen's exclamation rings in, the poem totally changes. The gas, so commonly used during the war, damages no one who got his mask on in time, but others weren't so lucky. Now the description is no longer of marching, but of death and dying. "Watch the white eyes writhing in his face, / His hanging face, like a devil's sick of sin; / If you could hear, at every jolt, the blood / Come gargling from the froth-corrupted lungs, / Obscene as cancer," and, thus, Wilfred Owen makes his readers highly aware of the atrocities of war.