Oh, I'd say it's because most of those leaders who can't let their countries live in peace are psycho communist or socialist despots with a tenuous (at best) grip on reality, like Putin. In other words, they don't look at the world the way a civilized person does, they look at the world the same way a criminal does, meaning they wonder what they can take and get away with it. That's why freedom loving countries have to maintain a strong defense (so that we don't have to go to war and use it.)