Wilfred Owen's "Dulce et Decorum Est" is one of the most famous examples of war poetry. Written between 1917 and 1918, Owens leads his audience through the foxholes and trenches of World War I to a famous line from the Roman poet Horace: dulce et decorum est pro patria mori ("Sweet and fitting it is to die for one's country"). Before we reach Horace's famous phrase, however, Owen shows his audience that warfare is anything but pleasant.
In the third line of the poem, Owens makes reference to "haunting flares." This is a reference to illuminating flares that at night were shot into the sky above a battlefield to cast light upon potential targets. For Owen's combatants, the light of the flares appears to be shining on the backs of the soldiers as they try to move away from the area of combat. Thus, Owen says that "on the haunting flares we turned our backs / And towards our distant rest began to trudge."
Thus, in Owen's poem, these flares provide a sort of weird and surreal light that casts a grotesque pall over the remainder of the poem.