Can a democracy change its mind? (The Mytilenean debate)

The place is Athens, the year 428 B.C. The war between Sparta and Athens has been raging on for about three years now. Pericles, Athens’s most influential politician, died the previous year in the plague that swept the city. The city is in disarray.

More disturbing news: one of the city’s closest allies and member of the Delian League (something like an ancient NATO), Mytilene, has decided to secede and side with Sparta. The Spartans have decided to help them, but the powerful Athenian navy, which has control of the seas, manages to capture Mytilene with the support of the local pro-Athenian faction.
The Athenian general contacts Athens: What shall we do with Mytilene?

The Athenians assemble to discuss this. The opinion of Cleon, a strident populist, expertly exploits Athenian anxieties and feelings of betrayal and ends up dominating in the debate: Mytilenians must die.

A ship is dispatched to Mytilene to deliver the orders. While the ship is on its way, Athenians strongly opposed to this decision demand a new debate, which is approved.

In this second debate, Cleon is furious and accuses Athenians of being victims of their own pleasure of endless debates in political matters. He urges the population to uphold the decision and not to be traitors to themselves: a decision has been made and we should stick to it.

Diodotus speaks next in support of the opposing view, arguing that haste and anger are the two greatest obstacles to wise counsel. Using Cleon’s arguments against him, he reframes the question: it is not about Mytilene’s guilt or whether it is right for Athens to seek vengeance, but about what is in Athens’s best interests. Would the proposed death penalty deter a potential future revolt and support the efforts of the Mytilenean pro-Athenian faction or would it further alienate Mytilenians?

At the end of the second debate, Diodorus’s rational arguments have swayed the opinion of the assembly towards sparing the general population of Mytilene and executing only the leaders of the revolt. A second ship is dispatched to deliver the new decision and annul the first one. Perhaps because the first ship was in no rush to deliver such grave news, the second ship arrives first and the Mytileneans are spared.

source: Thucydides, "History of the Peloponnesian War", book 3