The Lark (Symbol)
A lark is a small ground-dwelling songbird, famous for its melodious singing. In "Suicide in the Trenches," the young man (prior to becoming a soldier) whistles early in the day with the larks after having slept soundly through the night. The lark symbolizes the early morning, and the young man's innocent optimism.
The Bullet (Symbol)
The bullet that the young soldier puts through his brain is indicative of the violence that drives him to suicide. It is not the enemy's bullets, but one of his own that takes the young man's life. Bullets themselves are a weapon (meaning a tool for violence) used in war, and the young man's innocence is ruined by exposure to this violence.
Smug-Faced Crowds (Symbol)
In the final stanza, the speaker addresses the "smug-faced crowds" who cheer the soldiers on (Line 9). This represents members of the British public who supported the war and didn't acknowledge how terrible it was for the men who fought. The Allied propaganda machine contributed to the formation of the smug-faced crowds.
