The Queen's Army (The Lunar Chronicles, #1.5)

Note: this short story is also available in "Stars Above".

It is time. The boy must leave his family to serve in the Queen's army. To be chosen is an honor. To decline is impossible. The boy is modified. He is trained for several years, and learns to fight to the death. He proves to the Queen—and to himself—that he is capable of evil. He is just the kind of soldier the Queen wants: the alpha of his pack.