Wilfred Owen’s “Dulce et Decorum Est” graphically describes the horrors of war from his experience as a solder in World War I. He opens the poem with
Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge,
Till on the haunting flares we turned our backs,
And towards our distant rest began to trudge.
Under the weight of their gear, Owen and his fellow troops stagger through mud in retreat from German soldiers. As daylight fades, the Germans send small rockets over the front line to illuminate the area briefly. Lasting ten to fifteen seconds, light from the flares reveal any enemy soldiers still in No Man’s Land.
The imagery of "haunting flares" serves as a metaphor for battle. They are “haunting” for two reasons. First, they are inescapable—anyone caught in the light of a flare is exposed and then shot dead. Second, although the British soldiers physically leave the battlefield (at least temporarily), they still can see, hear, and feel the flares behind them. The men actually cannot escape from any reminders of the war or enemy no matter what they do.
“Haunting” also emphasizes the deathly aspect of war. The soon-to-be dead soldier in the poem who is unable to protect himself against gassing or chemical warfare will become a corpse. The memory of this soldier and other war casualties forever haunt Owen.
The flares represent the perversion of war. During normal times (i.e., peace), the light of flares can be viewed as celebratory or decorative. Flares also provide illumination for safety (as in flares lit around a road accident to keep victims safe from oncoming traffic). During wartime, however, flares are used to ferret out enemy troops to kill.
Finally, the verb “to flare up” emphasizes the act of eruption and spreading, like a conflict or illness. War is the conflict and political hatred is the illness. The maimed and slaughtered soldiers multiply like a rash that flares up and grows uncontrollable—supposedly in the name of patriotism.