A bayonet is a bladed weapon that can be attached to a firearm, like a rifle. Keith’s father used one in World War I, and he carries it with him when he goes off to the Home Guard on weekends. Keith claims that his father killed five Germans with the bayonet, but it’s more likely that he actually killed them with his pistol. In their privet hideout, then, Stephen and Keith pretend that they have a similar bayonet. In reality it is a carving knife without a handle, found in the rubble of Miss Durrant’s bombed house, but Keith sharpens it and the boys imagine it as a bayonet to the point that to them, it truly is a bayonet. They then swear an oath of secrecy on the bayonet when they start spying on Keith’s mother, and the object seems to give the oath great weight through both its importance and its inherent danger (as suggested by the “cut my throat and hope to die” part of the oath).
As Keith’s father’s weapon and the boys’ imagined treasure, the bayonet initially seems to represent bravery, military skill, and the nationalistic “glories” of war. Yet its role as an object of violence—Keith’s father uses his bayonet to torture his wife, and Keith in turn uses the boys’ “bayonet” to torture Stephen—undercuts these ideas, and shows how war is ultimately cruel, destructive, and anything but glorious. The bayonet thus represents the harsh realities of war being brought into the intimacy of the neighborhood, home, and family.