Editing 5th century BC (section)

===[[420s BC]]===
* [[429 BC]]: [[Battle of Chalcis]]—[[Chalcidians]] and their allies are defeated by [[Athens]].
* [[429 BC]]: [[Battle of Naupactus (429 BC)|Battle of Naupactus]]—[[Phormio]] defeats the [[Peloponnesian]] fleet.
* [[429 BC]]: An [[Plague of Athens|outbreak of a plague]] kills over one-third of the population of [[Athens]].
* [[429 BC]]: King [[Sitalkes]] of [[Thrace]] invades [[Macedon]]ia.
* [[428 BC]]: [[Mytilene]] rebels against [[Athens]] but is crushed.
* [[428 BC]]: [[Sparta]] attempts to crush a rebellion on [[Corcyra]], but cancels the effort when the Athenians try to intercept them.
* [[428 BC]]: The Greek colony of [[Cumae]] in [[Italy]] falls to the [[Samnites]].
* [[427 BC]]: The leaders of the Mytilenian revolt are executed.
* [[427 BC]]: [[Plataea|Platea]] surrenders to the Spartans, who execute over 200 prisoners and destroy the city.
* [[427 BC]]: The Athenians intervene in [[Sicily]] to blockade Sparta from the island.
* [[428 BC]]: The philosopher [[Plato]] is born.
* [[426 BC]]: [[Demosthenes]] unsuccessfully besieges the [[Ancient Corinth|Corinth]]ian colony of [[Leukas]].
* [[426 BC]]: When [[Ambracia]] invades [[Acarnania]], they seek help from the Spartans and Athenians respectively. The Athenians then defeat the Spartans in the [[Battle of Olpae]].
* [[425 BC]]: Demosthenes captures the port of [[Pylos]] in the [[Peloponnesus]].
* [[425 BC]]: The Athenians invade [[Sphacteria]] and defeat the Spartans in the [[Battle of Pylos]].
* [[424 BC]]: [[Sicily]] withdraws from the war and expels every foreign power. Thus, [[Athens]] is forced to withdraw from the island.
* [[424 BC]]: The Athenians try to capture [[Megara]], but are defeated by the Spartans.
* [[424 BC]]: The Spartan general [[Brasidas]] captures [[Amphipolis]], which is a setback for Athens. [[Thucydides]] is held responsible for the Athenian failure and is ostracised. This gives him time to start writing his [[History of the Peloponnesian War|history book]].
* [[423 BC]]: The Athenians propose a cease-fire, which the Spartan general Brasidas ignores.
* [[422 BC]]: The Spartans defeat the Athenians in the [[Battle of Amphipolis]], where the Athenian [[Cleon]] and the Spartan [[Brasidas]] are both killed.
* [[421 BC]]: The [[Peace of Nicias]] puts a temporary end to the hostilities between [[Athens]] and [[Sparta]].
* [[420 BC]]: [[Alcibiades]] is elected [[strategos]] of Athens and begins dominating Athenian politics.