The theme of "Suicide in the Trenches" is presented in the poem with the great contrast between society's romantic perception of trench warfare and its reality as exemplified by the young soldier. 

This theme of the stark reality of war is explored by means of diction and tone. In the first stanza, the young soldier has a romanticized view of life--grinning in "empty joy." In truth, he has no experience of the harsh realities of war. However, once he is sent to the war front where the World War I soldiers engage in battle from the trenches, this naive soldier becomes "cowed and glum," experiencing horrifying fear and carnage, along with having to spend long hours of being subjected to bitter cold, lice infestation, and starvation.

In the final stanza, the speaker's tone becomes very sardonic as he addresses those who believe war is glorious:

You smug-faced crowds with kindling eye
Who cheer when soldier lads march by,
Sneak home and pray you'll never know
The hell where youth and laughter go

In the last two lines of Siegfried Sassoon's poem, the speaker expresses his scorn for those who romanticize something as horrific as war, a hellish condition that leads a despairing, innocent lad to kill himself in a lonely trench thousands of miles from his home.