The battle had gone on for centuries.
The armies of the A would launch attacks. Then the armies of the B would launch attacks. The two sides would bunker themselves in, and the attacks rendered futile. No damage would be done, and no soldiers killed.
So then why is this war the deadliest the world has ever known? Perhaps it's because of its great length, or traitors, or something completely different.
What do I think: it's because the entire world and all people lie between A and B. But since they're part of neither, they're easy to kill without guilt.