History of the Peloponnesian War

For in all your cities the commonalty are now your friends and either revolt not with the few, or, if they be compelled to it by force, they presently turn enemies to them that caused the revolt; whereby when you go to war, you have the commons of the adverse city on your side.

But if you shall destroy the commonalty of the Mytilenaeans, which did neither partake of the revolt and as soon as they were armed presently delivered the city into your hands, you shall first do unjustly to kill such as have done you service, and you shall effect a work besides which the great men do everywhere most desire. For when they have made a city to revolt, they shall have the people presently on their side, you having foreshown them by the example that both the guilty and not guilty must undergo the same punishment.