Wilfred Owens' poem "Dulce et Decorum Est" (1920) is modeled on a poem by the Roman poet Horace entitled "Dulce et Decorum est Pro Patria Mori," which means "it is sweet and proper to die for one's country. Horace's original poem was a serious argument that a Roman should be willing to die for his country, but in the hands of Wilfred Owen, the sentiment becomes the basis of his strong indictment of war, in general, and World War I, in particular. Owens himself fought in the war and died in its trenches when he was 25. He wrote a letter to his mother shortly after he arrived in the trenches in France telling her that he was not "at the front, but in front of the front."
Aside from trying to record the horrors of war, Owens wrote this poem in part to counter an attitude within the civilian British population that sending young men to war was the equivalent of sending them into a rugby game. There was, at the time, a poet named Jessie Pope, who wrote pro-government literature, including poems, in which she encouraged British young men to enlist. One of her poems, in fact, is entitled, "Who's for the game?" The attitude expressed in this poem horrified Owens because it betrayed a complete lack of understanding of the horrific conditions and deaths that characterized WWI in France, and he decided the British public should understand what they were asking their young men to do.
Owens' choice of the central image--a man dying of poison gas--is absolutely perfect because it illustrates both the horror and the impersonality of death in the trenches:
But someone still was yelling out and stumbling/And flound'ring like a man in fire or lime,--/Dim through the misty panes and thick green light,/As under a green sea, I saw him drowning.
The soldier, whose face is visible through the panes of the green glass of his gas mask, is essentially drowning in his own bodily fluids after ingesting mustard gas, and there is absolutely nothing anyone can do to save him. For Owens, this image captures the futility of the war: the soldier is not dying as the result of a battle won or territory gained; he is simply a victim of an impersonal weapon against which he has no defense.
When, in the last stanza, Owens addresses the reader as "My friend," he is addressing the complacent British civilian who may think that war is the equivalent of a game. In Owens' view, however, the "game" is not just deadly but inhuman.