Thucydides, III 36 ff. Mitylene, the largest city on the island of Lesbos and one of the 'favored' cities in the Delian League revolted during the Peloponnnesian War. After a length siege, the city surrendered. The ringleaders were sent to Athens for trial. In a set of paired speeches, Thucydides explains the issues...

...  and after deliberating as to what they should do with the former, in the fury of the moment the Athenians determined to put to death not only the prisoners at Athens, but the whole adult male population of Mitylene, and to make slaves of the women and children. It was remarked that Mitylene had revolted without being, like the rest, subjected to the empire; and what above all swelled the wrath of the Athenians was the fact of the Peloponnesian fleet having ventured over to Ionia to her support, a fact which was held to argue a long-meditated rebellion. [3] They accordingly sent a trireme to communicate the decree to Paches, commanding him to lose no time in despatching the Mitylenians. [4] The morrow brought repentance with it and reflection on the horrid cruelty of a decree, which condemned a whole city to the fate merited only by the guilty.

At the assembly to reconsider: Cleon, son of Cleaenetus, the same who had carried the former motion of putting the Mitylenians to death, the most violent man at Athens, and at that time by far the most powerful with the commons, came forward again and spoke as follows:-- ‘I have often before now been convinced that a democracy is incapable of empire, and never more so than by your present change of mind in the matter of Mitylene. [2] Fears or plots being unknown to you in your daily relations with each other, you feel just the same with regard to your allies, and never reflect that the mistakes into which you may be led by listening to their appeals, or by giving way to your own compassion, are full of danger to yourselves, and bring you no thanks for your weakness from your allies; entirely forgetting that your empire is a despotism and your subjects disaffected conspirators, whose obedience is insured not by your suicidal concessions, but by the superiority given you by your own strength and not their loyalty.

... Our mistake has been to distinguish the Mitylenians as we have done: had they been long ago treated like the rest, they never would have so far forgotten themselves, human nature being as surely made arrogant by accomodation, as it is awed by firmness. [6] Let them now therefore be punished as their crime requires, and do not, while you condemn the aristocracy, absolve the people. This is certain, that all attacked you without distinction, although they might have come over to us, and been now again in possession of their city. But no, they thought it safer to throw in their lot with the aristocracy and so joined their rebellion! ... I therefore now as before persist against your reversing your first decision, or giving way to the three failings most fatal to empire--pity, sentiment, and indulgence. [3] Compassion is due to those who can reciprocate the feeling, not to those who will never pity us in return, but are our natural and necessary foe ... For if they were right in rebelling, you must be wrong in ruling. However, if, right or wrong, you determine to rule, you must carry out your principle and punish the Mitylenians as your interest requires; or else you must give up your empire and cultivate honesty without danger.

An otherwise unknown Athenian, Diodotus responds:

I have not come forward either to oppose or to accuse in the matter of Mitylene; indeed, the question before us as sensible men is not their guilt, but our interests. [2] Though I prove them ever so guilty, I shall not, therefore, advise their death, unless it be expedient; nor though they should have claims to indulgence, shall I recommend it, unless it be clearly for the good of the country. And I ask you not to reject my useful considerations for his specious ones: his speech may have the attraction of seeming the more just in your present temper against Mitylene; but we are not in a court of justice, but in a political assembly; and the question is not justice, but how to make the Mitylenians useful to Athens.

All, states and individuals, are alike prone to err, and there is no law that will prevent them; or why should men have exhausted the list of punishments in search of enactments to protect them from evil-doers? ...In fine, it is impossible to prevent, and only great simplicity can hope to prevent, human nature doing what it has once set its mind upon, by force of law or by any other deterrent force whatsoever. ...We must not, therefore, sit as strict judges of the offenders to our own prejudice, but rather see how by moderate chastisements we may be enabled to benefit in future by the revenue-producing powers of our dependencies; and we must make up our minds to look for our protection not to legal terrors but to careful administration....As for Cleon's idea that in punishment the claims of justice and expediency can both be satisfied, facts do not confirm the possibility of such a combination. ... this is the wisest course, and without conceding too much either to pity or to indulgence, by neither of which motives do I any more than Cleon wish you to be influenced, upon the plain merits of the case before you, be persuaded by me to try calmly those of the Mitylenians whom Paches sent off as guilty, and to leave the rest undisturbed. [2] This is at once best and most useful for the future.