The future of war is in the hands of the children. That's the premise of 'Ender's Shadow', a parallel book to 'Ender's Game'. In the not too distant future, humans have become a space faring race, and encounter a sentient species degratorily nicknamed 'Buggers'. The Buggers are a hive mind, reminiscent of 'Aliens' and 'Starship Troopers'. The encounter is acrimonious, and results in the extermination and colonization of several planets previously occupied by humans. Sort of like pest control, in reverse. The initial battles finally result in the defeat of the Buggers, after the destruction of the Hive Queen by Mazer Rackham. Both sides retire to lick their wounds, with neither side ever having discovered the other's home planet, for the administration of the final coup-de-grace.
The 'Ender' referred to in the title, is Andrew Wiggin, a prodigy in the art of space warfare. The powers controlling Earth have decided that the best way to gain an advantage over the enemy is to search through the children of Earth, via intelligence testing and other means, for a prodigy, the next Alexander, Napoleon, Von Clausewitz. Children meeting this criteria are sent to 'Battle School', where they play war games, in groups of 'toons', in null gravity rooms.
Ender's Game initially saw life as a short story, which I read in an anthology called 'Lightfighters'. Card took the story, and lengthened it into Ender's Game. In the story, there was a character called 'Bean', was was younger than Ender Wiggin, but possibly as or more intelligent and better at warfare than Ender. Ender's Shadow is Bean's story, of his life on the streets of Amsterdam, of his life in Battle School, of his view of the boy genius Ender Wiggin, on whom all human kind have placed their hopes of winning the war against the Buggers.
Ender's Shadow, is, as Card says, a parallel story. It tells the story of Ender's Game, but from the viewpoint of a very different pair of eyes. Bean is a genetically manipulated human child, whose initial conception was in contravention of international law forbidding genetic experimentation. Designed to be a genius, he survives the mass culling of his fellows in the genetic lab, and survives on the streets of Amsterdam using a mixture of guile, cunning, and the ability to read at 3 years of age.