The Mytilene oligarchy of Lesbos forces a…

The Ionian city of Colophon rebels between 430 BCE and 428 BCE against Athens; Pissuthenes, Persian satrap of Lydia from at least 440 BCE, sends mercenaries in support of the rebels.

Archidamus leads a third Spartan invasion of Attica in 428 BCE.

...Sparta shrewdly summons the Mytileneans and other injured Greeks to the Olympic Games, thus emphasizing that one aspect of the war is the tension between Dorians and Ionians. (While Athens is hardly formally excluded from the solemnities, Olympia has always had a Dorian flavor.)

Alcidas, the Spartan commander sent to assist the Mytileneans, fails, however, to do anything for them.

On its surrender (427), the city narrowly escapes the wholesale executions of its citizens and enslavements of women and children that Cleon—whose decree passes—recommends, but only as a result of second thoughts, the next day, on the part of the Assembly (these events and decisions form the context of the famous “Mytilene debate”).

It is to the Athenians' credit that some of them are moved by the thought that their original decision was bloodthirsty, and rescind the decree.

The Athenians kill only the leaders of the revolt.

The Spartan conquerors of Plataea, on the other hand, have no such doubters among them.

When the remaining Plataeans surrender (some had already broken out to Athens) in 427, the Spartans put them to death to a man, after posing the “brief question”, “Have you done anything for Sparta during the war?”

Thebes takes over Plataea's vote in the Boeotian League and some of its territory after the destruction of the city in 427 BCE.

Civil strife occurs at Corcyra when, after a bloodbath in 427 BCE, the democratic pro-Athenian faction prevails over the oligarchical pro-Spartan party, with the Athenian commander Eurymedon not attempting to stop it.

The enslaved people, promised freedom by both sides, go over to the democrats (who may simply have made better offers).

Athens mounts a vain attempt by the Athenian general Nicias to take Megara by military means in 427, then ...

…seizes all but one base in the Gulf of Corinth.

Naxos allies with …