In Chapter 15 of Ender's Game, how is Graff acquitted?
In Chapter 15, where we find out that in spite of the massive public outcry against Graff and the military for what they did to the children and particularly to Ender, Graff has been acquitted, Graff reveals how he managed to ensure that he was acquitted. This, as he reveals, was through simple revelation of the truth: Ender would not have won the war without the training that he had received, as the Graff describes:
I said I did what I believed was necessary for the preservation of the human race, and it worked; we got the judges to agree that the prosecution had to prove beyond doubt that Ender would have won the war without the training that we gave him. After that, it was simple. The exigencies of war.
What Graff does to assure his acquittal therefore is to point to "the exigencies of war," or, in other words, what he had to do in order to secure victory for mankind meant that difficult choices and terrible sacrifices had to be made. Ender's life and the victims of those he harmed and killed were just one of those necessary sacrifices in order to secure victory. When the prosecution were not able to prove that Ender would have secured victory without the training Graff provided, it was clear that there was no case against him. As objectionable as what Graff did, it is also clear that he was vital in helping the human race to survive.