In Thucydides’ account of the Mytilenian debate, the citizens of Athens are discussing whether they should put to death all Mytilenian men and enslave all women and children. Mytilene, a former Athenian ally, had revolted against Athenian oppression but lost in a fight against the Athenian army.
In a previous debate, the Athenians already decided to punish Mytilene as described above. Now, the Athenians meet again to discuss whether they should take back that decision or not. Thucydides recounts the speeches of two very influential Athenian citizens: Cleon, and Diodotus.

First, Cleon argues for the death penalty. He brings forward the following arguments:

1. [Argument questioning the decision-making ability of the council]: The citizens gathering in the council are merely interested in hearing a good speech than in really thinking about the arguments for and against the issues to be decided. They are biased to favour original arguments and rhetorically talented speakers. Instead they should think in terms of what really is in Athens interest. [Remark: This would of course be Cleon’s following arguments (except of the one argument that appeals to justice instead of interests)]
2.  [Second argument questioning the decision-making ability of the council]: The influence of Athens over its partners resembles a tyranny that the partners do not like. Therefore they are always secretly plotting against Athens to find ways to get out of that oppression. Democracy is ill-suited to manage these complicated external relationships, because “fear and conspiracy play no part in [the democratic citizens’] daily relations with each other” (Brown et al., p. 45), but fear and conspiracy are exactly the most prominent features of interstate relationships.
3.  [Argument referring to Athens’ leadership role]: Athens’ leadership over others only works because of Athens’ superior strength, and not because of the goodwill of others. Just like domestic politics, interstate politics are best modelled with clear rules that are strictly followed. If Athens took back its previous decision to destroy Mytilene, Athens would show weakness and thus weaken its leadership role. Even just discussing and hesitating to execute the punishment are damaging Athens’ leadership status.
4. [Argument referring to justice]: Mytilene deliberately chose to attack Athens. This attack was unprovoked and unnecessary, since Mytilene enjoyed a solid level of welfare as an Athenian partner. The attack stemmed from the arrogance to believe that Mytilene would stand a chance against the Athenian army. Therefore, the hard punishment is just.
   Moreover, the attack was executed by the people as a whole: Although it had been the aristocrats who initiated it, the ordinary people did not take the chance to stand up against the aristocrats’ decision. The ordinary people thus failed to stop the revolt. Consequently, it is just to apply the punishment to the whole of the Mytilenian population.
5. [Argument referring to deterring Athens’ allies]: The sentence for Mytilene will serve as precedence for the other Athenian allies. When deciding to revolt, these allies usually have to consider the following possible outcomes: First, to win and to gain freedom; second, to loose and to be punished. If the sanctions imposed by Athens on an unsuccessful revolt were not very dreadful, the incentives to risk a revolt would rise. This would be counter-intuitive to Athens’ interests.

After that, Diodotus brings forward the following arguments why the decision to destroy the Mytilene population should be taken back:

1. [Argument questioning the previous decision]: Decisions should not be made in “folly” and “anger” (p. 49). The decision to destroy Mytilene’s population was guided by anger.
2. [Argument supporting the decision-making ability of the council – countering Cleon’s first argument]: It is “intolerable” to say that speakers and citizens in the council are biased towards original arguments and rhetorically good speakers. The council is the right decision-making authority and competition between speakers is the right mode to bring about decisions.
3. [Argument questioning the deterrent impact of death sentence]: The death sentence also does not hinder criminals to commit crimes that are punished with execution. The same applies to states. People and states will take action when they see a well-enough chance to be successful, disregarding of the punishment if they fail.
4. [Argument referring to further use of allies who revolted and failed]: If Mytilene is destroyed, Athens cannot request tributes as reparation for the failed revolt. These tributes are the basis for Athenian power.
5. [Argument referring to Athens’ costs of fighting revolts]: If revolutionaries face death after losing, they will make more careful preparations for revolt and they will fight to the bitter end. If they can count on a milder penalty, they are more likely to surrender and thus lower the costs of the siege.
6. [Second argument referring to Athens’ costs of fighting revolts]: If citizens of other allied states knew that the whole population would be punished after a failed revolt, they will fully back their leaders decisions, since they will be punished no matter if they actively took part in the revolt or not.
7. [Argument referring to prevent revolts in the first place]: It would be better to treat allies as well as possible to not make them want to revolt in the first place. [Note: Actually this has nothing to do with the decision how to punish Mytilene)
8. [Argument referring to justice]: Not all Mytilenians actively took part in the revolt. It would be unfair to punish the whole population.

Personally, I think that both speakers are making a good point. However, I believe that Cleon’s overall argument is more persuasive.
As regards the arguments to justice, both Cleon’s and Diodotus’ arguments are convincing, but I am slightly more on Diodotus’ side to argue that not all Mytilenians took part in the revolt. To punish the entire population would have been like sweepingly executing or enslaving the entire German population after the Second World War. Individual punishments for the leaders and for people who have proven to commit war crimes are more just than collective punishment.

Anyhow, I think that the arguments concerning the interests of Athens weigh heavier than the arguments concerning justice; and I think that in this case Cleon’s arguments are more convincing. Especially his point that taking back strict punishments is weakening Athens’ leadership role is important. I probably would have favoured Diodotus’ overall argument in the first debate, but after the decision has been made, Athens should stick to it. Otherwise Athens would give the impression of being weak and internally fighting, which is a strong short-term incentive for other colonies to take their chance to revolt.
Cleon argues that lack of a deterrent will raise incentives to revolt, because others may feel that the current Athenian government will not severely punish revolts. Combined with the previous notion, this may add up to a deadly cocktail for Athenian power: Colonies could conclude that a) Athens is temporarily weak (i.e. the risk of losing a revolution is low) and b) that even if a revolution is lost, the sanctions would be bearable. It would then possibly be sufficient for just one or two allies to take this “now or never” chance to start a revolution, and the other colonies would follow. Sooner or later, all or most Athenian allies could join the revolution and put Athens entire existence at risk. This risk is way too dangerous to still take into account second-priority values like justice.

Finally, I can understand why the Athenian citizens closely voted to follow Diodotus. It was a tactically flawed move of Cleon to question the decision-making ability of the council, because by nonetheless voting for Cleon, it would have acknowledged its own [partial] incompetence as a part of Cleon’s entire argument. In contrast, it was very clever of Diodotus to link an appraisal of the competency of the council to his argument. Moreover, by claiming that the will to punish Mytilene was just an expression of temporal [and thus excusable] emotions like anger, Diodotus paved the way for any citizen who supported Cleon in the first discussion to take back their decision.

Sources:

• Thucydides: History of the Peloponnesian War