Ender's Game starts out on the world of earth. After 2 vicous wars with an ant-like species called the Buggers Earth's defenses have been stretched thin. On Earth only two Children are allowed to be born per family. But the Wiggin's have had a third, by military order. Andrew Wiggin self appointed the name Ender, was chosen to be born as the next great commander of the war, unknown to him. Throughout his training in battle school he is confronted with challenge after challenge after challenge. He over comes them all and while maintaining the highest scores ever seen in battle school, he becomes the youngest commander of an army, an army with the reputation to be the worst. But even with all the odds stacked against him he again prevails over all odds. Later he is whisked away to command school where he begins training in a simulation of a fleet, that later he discovers is the actual fleet.
And without knowing that the fleet was real he did the unthinkable, he used a weapon of mass destruction, the M.D. device. A device that destroys everthing in its path. He used the M.D. device against the Bugger home world hence ending the war.