How are the horrors of war expressed in "Dulce et Decorum Est"?
I assume that you are referring to "Dulce et Decorum Est" by Wilfred Owen, his most famous poem, and I have edited the question accordingly. Please remember in future to provide the title of the work you are referring to.
From the very opening words of this poem it is clear that Owen is playing with the image that his audience had of war and the actual brute reality of what fighting in WWI was like. Consider the first two lines of this incredible poem:
Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge...
War is shown to have transformed these young men into "beggars" and "hags" as they "curse" their way through the sludge of the battlefield. There are no romantic images here of war that indicate that it is something noble. Again and again in the first stanza images are supplied that show war as something that is dehumanising and something that strips away, rather than bestows, dignity, on the soldiers who are unfortunate to be caught in its path. Consider expressions such as "Men marched asleep," "blood-shod," "All went lame; all blind," "Drunk with fatigue." Clearly the man are beyond physical exhaustion and are shown to be walking like zombies, desperate to reach their rest.
Owen of course doesn't stop there in evoking the horrors of war. By presenting us with a disturbingly graphic image of one solder dying because of a gas attack, focusing on his "froth-corrupted lungs" and the "white eyes writhing in his face." Juxtaposing this event with what Owen calls "the old lie" he issues his challenge: it is not sweet and noble to die for one's country, and those who spread this lie to "children ardent for some desperate glory" are incredibly wrong.