One of the definite strengths of this poem is the way that it sets us up with definite expectations about what to expect because of its title, and then presents us with the exact opposite. With such a title, we expect a poem exploring the glory of soldiers and their bravery as they commit valiant deeds. However, note how soldiers are presented in the first two lines of the poem:
Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge...
The brave and noble soldiers we expect to meet are presented as dehumanised wrecks of human beings, with descriptive words such as "old beggars" and "knock-kneed" presenting them as old men, already half-dead. "Hag" is a term used to describe a very old woman who is normally not very attractive, so this word adds to the presentation of the soldiers. Of course, such descriptive words help convey the theme of the poem: that it is anything but sweet and noble to die for your country.