Lesbos had sided with Athens' Delian League since the beginning of the war, providing it ships. They had a falling out, and collaborating with Sparta, the Mytileneans revolted.
Kleon, having risen to power following the death of Perikles, took it upon himself to deal with the revolt, intending to wipe out the Mytileneans completely.
The Athenians were given the chance to vote for the fate of Lesbos, and Kleon attempted to manipulate the vote to kill the population. Due to the efforts of the Spartan misthiosKassandra and her friends, especially Sokrates and Aristophanes, they were able to weaken Kleon's support and shift the vote. Instead of killing the population, clerouchs ('settlers') were sent to occupy the island.