There are two major themes in this poem. The first is the horrors of trench warfare. This is brought forth in many specific physical details such as the bootless men marching “blood shod”, being “lame”, “coughing like hags”, etc. The general exhaustion and injury is made worse by the gas attack, and the extreme pain endured by the people assaulted by it.
The second theme is the contrast of the reality of impersonal trench warfare and the actual physical details of soldiers' deaths with the epic literary tradition of noble individual combat (“you would not tell with such zest …”). Owen says that no one who has experienced gas attacks or seen people dying in these horrible ways (“gargling from froth corrupted lungs”) would write of war so admiringly.