I wish to add further elements to the discussion about the essence
of Ender's Game.
The story is about a child being turned into an adult way too early. But it is also about being alienated.
Ender is not like the other kids that are recruited to the IF. He is sharper, and way more intelligent; and last, but not least, he is younger.
He is always regarded as something less than the other kids in battle school, because he is several years younger than the others.
The commanders of the school see this alienation, and ignore it, knowing that it will turn Ender into a more efficient soldier/commander than he would ever be were he to rely on others.
I believe that this book is also about being alienated, and having to offer more of yourself, trying to win some respect for the human being that you are. It is a story that, (in my humble opinion), tells you that for some people the only way to be acknowledged is to excel in what you do, and be so good at it, that no one can question you.
It is also a story about giving up your humanity. That is really what Ender is forced to do in order to defeat the buggers. There is no man in the IF who is capable of destroying an entire race in one swift blow. That is the reason they elect Ender to the role, and they shape him, and drive him on, just for this single purpose; being so separated from humanity, that you can destroy an entire race.
We see this same separation from humanity among statesmen all over the world. The ability to disconnect or disregard every emotion you have for other human beings, in order to act on behalf of the state. And I believe this is a major theme in this story, alienation and separation from humanity.