Ender's Game

Ender and his two siblings are geniuses. They have what the military needs to be able to combat a hostile alien civilazation bent upon the destruction of Earth. The aliens have already launched one attack and nearly overcame Earth's defence at the time. Now the idea is to be offensive rather than defensive and bring the war to the alien's home planet. Ender, a young boy, is trained to become a fleetcommander. His education is very brutal, but sacrifices must be made with the world at stake and Ender's training is rushed in order to be able to attack the aliens before they can lauch an assault of their own.
This book is easily one of the best science fiction novels that I've read, and it will probably come to be regarded as a classic.