Chapter 6 of Lord of the Flies begins with a description of a dark night on the island during which the children are restless and the littluns occasionally cry out with fear.  Soon, the quiet of the night is disrupted:

A sign came down from the world of grown-ups, though at the time there was no child awake to read it.  There was a sudden bright explosion and corkscrew trail across the sky; then darkness again and stars.

Obviously, readers understand that the "sign" is actually the explosion of a plane--and that the impact has killed the parachutist who attempted to eject.  Later in the chapter, the twins see the dead parachutist and think it is the beast.

Golding's underlying purpose in writing Lord of the Flies is to show us that we live in a corrupt world and that human nature is inherently evil--and therefore responsible for many of the troubles that we as a society encounter.  The fact that the adults are at war underscores Golding's purpose.  Golding makes the same point at the end of the novel, when a naval officer interrupts his man-hunt to rescue the boys.