Why did O'Brien's perspective change?

After returning home to the States, O'Brien feels utterly used. He had been fed lies about the military's purpose oversees. After participating in Major Callicles' investigation, however, he learns that the government made many concessions. O'Brien never had a choice to no participate, but he feels like he was owed something for his service. His innocence was taken from him. His peace of mind and restful nights were taken from him. Now he holds the government responsible for using thousands of men to fight a war which never needed to exist.