While they are in preliminary negotiations at "attack" begins. An attack that the sensors do not detect any weapons being used or any damage. Spock realizes that the war is being waged by computer. The buildings and infrastructure survive. Casualties are recorded and expected to report to a suicide station within twenty four hours. And the citizens comply. The head of the ruling council explains that they've accepted that they are barbarians and murderers. The people die at the rate of one to three million a year but the planet and the "civilization" survives.
When the Enterprise is declare a casualty Kirk does what he usually does when the prime directive is involved. Changes the rules. He and Spock throw several monkey wrenches into the works and destroy the war computers. Kirk admits that yes we're killers with generations of blood behind us but we don't have to kill anyone today. That's how it starts. We don't have to kill anyone today. And we'll worry about tomorrow when tomorrow comes.
Faced with fighting a real war with real weapons the two worlds are open to the idea of negotiating an end to the war. Kirk points out to the others that a real war wouldn't cause any more casualties than they've already suffered.
This is one of the scariest TV episodes or movies I have every seen. As I said keep your zombies.