Cleon was an Athenian politician and general in the second Peloponnesian War
Unfortunately the two largest sources for Cleon are Thucydides and Aristophanes, two men who had personal reasons to dislike him. Thucydides, was too late to save Amphipolis from the Spartans, and Cleon led the force that had him ostracised. Cleon also tried to prosecute Aristophanes, accusing him of slandering the city in front of subject allies, but really because Aristophanes insulted Cleon in front of the citizens.
His earliest act in politics that we hear of was in 431B.C. when he opposed Pericles' strategy of not fighting the invaders but instead staying behind the city walls and relying on the navy. However Pericles had too much power for Cleon to change the strategy. Cleon next appeared in 427B.C. leading one side of the Mytilene debate. The Mytilene debate took place after the Lesbian Revolt, Cleon advocated killing all the men and selling the women and children into slavery. The Athenians passed this although it was repealled the next day; this shows the violence of Cleon's character.
In 426B.C. Aristophanes wrote Babylonians, and Cleon attempted to prosecute him.
Cleon's most significant act was during the Pylos campaign in 425B.C. Demosthenes had encircled the Spartans on Sphacteria but the other generals were unwilling to move in and capture them. Cleon was not a general at the time and heckled the generals in the assembly. Nicias said that if Cleon thought he could do a better job he could take his generalship. Cleon accepted it and said he could do it in under twenty days. He kept his word. This was the most successful campaign for the Athenians in the Archidamian War. The same year he was involved in raising the pay for jurors from two to three obols a day. Cleon is also thought to be behind the decree reassessing the tribute collection from the allies, since it was instigated by Thoudippos, who was Cleon's son-in-law.
In 424B.C. the Spartan Brasidas captured Amphipolis, (Thucydides arriving too late to save it). Then in 423B.C. Cleon punished the citizens of Scione by executing the men and enslaving the women and children.
In 422B.C. Cleon led an expedition to counter the actions of Brasidas in Thrace. He took back several towns, and finally engaged Brasidas at Amphipolis. At Amphipolis both Cleon and Brasidas died, Thucydides implies Cleon was killed running away, although this is taken with a pinch of salt considering his dislike for Cleon.
Cleon was the first and biggest of the demagogues, he is noted for the violence of his character and his nasal voice. He was also repeatedly mocked about his father being a tanner. Cleon is often criticised for appealing to the lowest common denominator, however he was a successful general and a skilled orator, and if he was as bad as others made out it is unlikely that he would come to dominate Athenian politics as his did.