Cleon, a famous Athenian demagogue in the time of the Peloponnesian war, for about six years the leader of the party opposed to peace. Originally a tanner, he gradually abandoned his business, and became the champion of popular rights, a position for which his fluent speech and loud voice fitted him admirably. He first became prominent about 427 B.C. by his advocating the putting to death of the Mytilenean prisoners, but his first great success was the reduction of the island of Sphacteria, in which a Lacedæmonian force had long held out. It is by no means clear how far this exploit was not really due to the skilful dispositions of his colleague Demosthenes, but Cleon himself was highly elated with his success, and many of his countrymen must have credited him with military genius, for in 422 he was sent to oppose Brasidas, the great Spartan general, in Macedonia and Thrace. But for this task the demagogue was insufficient, and he only saved his character by falling in the battle fought under the walls of Amphipolis.
Our picture of Cleon has had the misfortune to have been painted in unfavourable colours by such masters as Thucydides and Aristophanes; and, as Grote has pointed out, it should be remembered that the great historian was full of oligarchical prejudices, while we know that Aristophanes girded at other men about whose nobility of character there is no doubt at all. Besides, it appears not unlikely that both owed the demagogue a private grudge; the former for having been banished at Cleon's instance while holding a subordinate command, the latter because Cleon had complained to the senate that in his comedy, The Babylonians, Aristophanes had ridiculed his country's policy in the face of foreigners, and that, too, in time of war. Whether just or unjust the picture, Cleon will live in the Knights of Aristophanes as the shiftless and unscrupulous demagogue, lying and pandering to the mob, which he befools for his own selfish ends. In this comedy he is one of the actual dramatis personæ, and this part the author played himself, not being able to find an actor bold enough to take it.