Lord of the Flies was published in 1954, less than a decade after the end of World War II. This war was effectively ended when America dropped two atomic bombs on Japan, and the implications of atomic warfare thus became a worldwide threat, forever changing the possibilities of wartime engagement. This led to the Cold War, a time of great tension as various countries, but particularly the United States and the Soviet Union, assimilated a massive arsenal of nuclear weapons.
Just after Piggy and Ralph find each other after the plane crash, Piggy asks, "Didn’t you hear what the pilot said? About the atom bomb? They’re all dead." Before the plane crashed, the boys overheard a troubling comment about the world they have left behind via the plane. It seems that the children were being evacuated somewhere, likely because of the threat of nuclear warfare. In the midst of their escape, someone has used an atomic bomb, which has killed an undefined "they." It isn't clear who has been killed, but Piggy believes that because of the nuclear devastation, there is no one at the airport who will know where they are. Therefore, they are truly lost on this island with no adults remaining who could possibly determine their location.
Placing the novel in the context of nuclear devastation is a reminder of the tendencies of humans to engage in acts of violence, destroying the natural world and other innocent beings in their quest for dominance.