"Dulce est decorum est / pro patria mori," the last line of the poem, means "It is sweet and fitting (or glorious) to die for one's country." The entire poem--a double sonnet--however, does not speak of the sweetness or glories of war. Instead, it speaks of the concrete realities Wilfred Owen encountered as an army lieutenant in World War I.
Owen was wooed to war by the type of war poetry popular before that war, poetry which focused on Duty, Honor, Country, and appealed to young men's desire to be heroes, to wear flashy uniforms, to win medals, to come home in glory. He even wrote a ditty in 1914 before he left for the front:
How meet it is and passing sweet
To die in war for others.
But sweeter still and far more meet
To die in war for brothers.
He'd bought into the propaganda every country uses to recruit young men for battle. Only after serving on the front and later spending time in a mental hospital for shell shock did he begin to write poetry that told the harsh realities of war.
"Dulce et Decorum Est" begins with this image:
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.
This is not the image of upright, energetic young men full of the fighting spirit and enjoying their esprit de corps. It likens them--young men, many still teenagers--bent double like beggars, broken, exhausted, sick and slogging through mud, by now immune to the fear of the flares and explosions around them, knowing they still had miles to go before they slept (to borrow a line from Frost). War, he is asserting, is not about medals and glory, but about the everyday realities on the ground, and those realities are ugly.
He goes on to tell of an incident in which he watched a man fail to don his gas mask in time, and the rest of the men stood there and, through the pallid green of their own masks, watched him die. He still sees that man, he writes: "In all my dreams, before my helpless sight, / He plunges at me, guttering, choking, drowning." The death he saw on the battlefield wasn't linked in any way to honor and courage and gallantry, as he expected; rather, it was random, meaningless, and pointless.
After describing how the man looked and sounded as they walked behind the wagon they flung the doomed man into, he finishes with barely-concealed anger. "If you could see him," he says, "...if you could hear...," "you would not tell with such high zest / To children ardent for some desperate glory / The old Lie...." He wants to put a stop to people using the Lie that it's a beautiful and wonderful thing to die for your country, because it's what we use to get ignorant and innocent young men to "volunteer" to die ugly, pointless deaths without honor or glory.