You see, the folks back home like to believe there is an order in war. A sense of right and wrong. Some things are allowed, some things are not. That's why civilian leaders on the home front wanted to court-martial Lieutenant Calley. But My Lai was just part of the war, not some isolated incident.
War has no rules.
No time outs. No penalties for fouls. If you can't accept that, then you can't accept war, because that's what war is. This is what O'Brien is saying. War has its own reality.