From TOS, "A Taste of Armageddon", another one of Kirk's greatest speeches [
transcript]:
ANAN: You realise what you have done?
KIRK: Yes, I do. I've given you back the horrors of war. The Vendikans now assume that you've broken your agreement and that you're preparing to wage real war with real weapons. They'll want do the same. Only the next attack they launch will do a lot more than count up numbers in a computer. They'll destroy cities, devastate your planet. You of course will want to retaliate. If I were you, I'd start making bombs. Yes, Councilman, you have a real war on your hands. You can either wage it with real weapons, or you might consider an alternative. Put an end to it. Make peace.
ANAN: There can be no peace. Don't you see? We've admitted it to ourselves. We're a killer species. It's instinctive. It's the same with you. Your General Order Twenty Four.
KIRK: All right. It's instinctive. But the instinct can be fought. We're human beings with the blood of a million savage years on our hands, but we can stop it. We can admit that we're killers, but we're not going to kill today. That's all it takes. Knowing that we won't kill today. Contact Vendikar. I think you'll find that they're just as terrified, appalled, horrified as you are, that they'll do anything to avoid the alternative I've given you. Peace or utter destruction. It's up to you.