The book is based on a prediction of nuclear weapons of a more destructive and uncontrollable sort than the world has yet seen. It had appeared first in serialised form with a different ending as A Prophetic Trilogy, consisting of three books: A Trap to Catch the Sun, The Last War in the World and The World Set Free.
A frequent theme of Wells's work, as in his 1901 nonfiction book Anticipations, was the history of humans' mastery of power and energy through technological advance, seen as a determinant of human progress. The novel begins: "The history of mankind is the history of the attainment of external power. Man is the tool-using, fire-making animal. . . . Always down a lengthening record, save for a set-back ever and again, he is doing more." The novel is dedicated "To Frederick Soddy's Interpretation of Radium," a volume published in 1909.
Scientists of the time were well aware that the slow natural radioactive decay of elements like radium continues for thousands of years, and that while the rate of energy release is negligible, the total amount released is huge. Wells used this as the basis for his story.
