Earth has been attacked and was almost defeated before humans repulsed the alien invasion. Now the Earthians are taking the battle to the alien home world. Their psychology has discovered that when it comes to war fighting using weapons not requiring brawn to operate, that children make the best warriors as their nervous systems are more flexible and adaptive. So an all child army is being raised to fight the aliens.
True to form, the adults are endlessly manipulative and the protagonist, as he is groomed for leadership, becomes increasingly aware that the true enemy may be these adults. His hunch is confirmed when the adults trick him into annihilating the alien home world.
The sense of compassion which enables Wiggins to outwit his enemy is also what leads him to love and understand them. In the case of the aliens, he feels obligated to make amends and find a way to continue their lineage. The Earthians make him an admiral at this point, a way of saying he's on his own.