The central theme of A Boy At War is coming-of-age, the process by which the mind changes from its focus on frivolous topics and ideas to real-world topics, mortality, and adulthood. The protagonist, Adam, is caught in the terrible events of the 1941 attack on Pearl Harbor; his world, which has focused on school, friends, and other childhood issues, changes to focus on World War II, the meaning and ideals behind the war, and the possibility that his father has been killed in the attack. Before this event, he never thought about these adult issues; now he finds that he must face them head-on. This is indicated before the novel even starts:
Nobody, however young, returns from war still a boy.
--Samuel Hynes, Flights of Passage: Reflections of a WWII Aviator
(Epigraph, A Boy At War by Henry Mazer)
Adam, like the young boy of Hynes's quote, leaves the war heading for adulthood. Although he does not fight in this particular conflict directly, it affects him as powerfully as it affects anyone involved; he discovers that his childhood concerns are smaller and less important than he had always assumed, and that he suddenly has to deal with adult concepts of mortality, death, and nationalism. This changes his ideals and his concerns; Adam is forced by circumstance to "grow up," and although he does not complete the journey by the end of the book, he finds himself changed in ways from which he can never recover.