Ender's Game by Orson Scott Card
This 1985 novel eventually spawned an 11-book series, but the original is still the best. Set in the future, when aliens have attacked humankind twice, the story revolves around young Ender Wiggin. As a child, Ender is taken by the government to learn the art of war through a series of games. It quickly becomes clear that Ender is a genius in his field. The story follows him through his military training, culminating in a final exam that may not be what it seems.